diff --git a/backend/api.http b/backend/api.http
index 8a0f9e2..9b04987 100644
--- a/backend/api.http
+++ b/backend/api.http
@@ -230,3 +230,36 @@ Content-Type: application/json
 
 ### Notu sil
 DELETE {{baseUrl}}/notes/BURAYA-NOT-ID
+
+# ─────────────────────────── Bütçe ve özet ───────────────────────────
+# Gruplar: needs · wants · culture · unexpected · tutarlar kuruş
+
+### Dönem özeti: gelir − sabit gider − birikim − devreden borç + devreden artı = harcanabilir
+GET {{baseUrl}}/periods/BURAYA-DONEM-ID/summary
+
+### Bütçe: 4 grup, aylık + haftalık döküm (haftalar gün sayısına göre)
+GET {{baseUrl}}/periods/BURAYA-DONEM-ID/budget
+
+### Aylık bütçeleri kaydet (4 grup birlikte)
+PUT {{baseUrl}}/periods/BURAYA-DONEM-ID/budget
+Content-Type: application/json
+
+{
+  "needs": 1500000,
+  "wants": 600000,
+  "culture": 200000,
+  "unexpected": 200000
+}
+
+### Bir haftayı elle ayarla (week_start = haftanın ilk günü); "amount": null → otomatiğe döner
+PUT {{baseUrl}}/periods/BURAYA-DONEM-ID/budget/weeks
+Content-Type: application/json
+
+{
+  "group": "needs",
+  "week_start": "2026-12-07",
+  "amount": 400000
+}
+
+### Bütçe önerisi — method: default (varsayılan yüzdeler) | previous (önceki dönem) | avg3 (son 3 dönem)
+GET {{baseUrl}}/periods/BURAYA-DONEM-ID/budget/suggestion?method=default
diff --git a/backend/src/routes/budget.rs b/backend/src/routes/budget.rs
new file mode 100644
index 0000000..0d10c02
--- /dev/null
+++ b/backend/src/routes/budget.rs
@@ -0,0 +1,246 @@
+//! Bütçe ve özet uç noktaları:
+//!
+//! | Yöntem | Yol | İş |
+//! |---|---|---|
+//! | GET | `/api/periods/{id}/summary`                     | gelir/gider/birikim/devir → harcanabilir |
+//! | GET | `/api/periods/{id}/budget`                      | 4 grup: aylık + haftalık döküm |
+//! | PUT | `/api/periods/{id}/budget`                      | 4 grubun aylık tutarları |
+//! | PUT | `/api/periods/{id}/budget/weeks`                | bir haftayı elle ayarla / otomatiğe döndür |
+//! | GET | `/api/periods/{id}/budget/suggestion?method=…` | `default` · `previous` · `avg3` |
+
+use axum::{
+    Json,
+    extract::{Path, Query, State},
+};
+use mongodb::bson::Uuid;
+use serde::{Deserialize, Serialize};
+use ts_rs::TS;
+use validator::Validate;
+
+use crate::{
+    auth::AuthUser,
+    error::{AppError, AppResult},
+    extract::ValidJson,
+    models::group::Group,
+    services::budget::{self, GroupBudget, SuggestMethod},
+    state::AppState,
+};
+
+/// Gruplara göre tutarlar (kuruş)
+#[derive(Serialize, Deserialize, Validate, TS, Clone, Copy)]
+#[ts(export)]
+pub struct GroupAmounts {
+    #[ts(type = "number")]
+    needs: i64,
+    #[ts(type = "number")]
+    wants: i64,
+    #[ts(type = "number")]
+    culture: i64,
+    #[ts(type = "number")]
+    unexpected: i64,
+}
+
+impl From<budget::GroupAmounts> for GroupAmounts {
+    fn from(a: budget::GroupAmounts) -> Self {
+        Self {
+            needs: a[0],
+            wants: a[1],
+            culture: a[2],
+            unexpected: a[3],
+        }
+    }
+}
+
+impl From<GroupAmounts> for budget::GroupAmounts {
+    fn from(a: GroupAmounts) -> Self {
+        [a.needs, a.wants, a.culture, a.unexpected]
+    }
+}
+
+#[derive(Serialize, TS)]
+#[ts(export, rename = "PeriodSummary")]
+pub struct SummaryDto {
+    #[ts(type = "number")]
+    income_total: i64,
+    #[ts(type = "number")]
+    fixed_expense_total: i64,
+    #[ts(type = "number")]
+    savings_goal: i64,
+    /// Önceki dönemden devreden borç
+    #[ts(type = "number")]
+    carryover_debt: i64,
+    /// Önceki dönemden devreden artı bakiye
+    #[ts(type = "number")]
+    carryover_surplus: i64,
+    /// gelir − sabit gider − birikim − borç + artı
+    #[ts(type = "number")]
+    spendable: i64,
+    /// 4 grubun aylık bütçe toplamı
+    #[ts(type = "number")]
+    budget_total: i64,
+    /// harcanabilir − dağıtılan (negatifse fazla dağıtılmış)
+    #[ts(type = "number")]
+    unallocated: i64,
+}
+
+#[derive(Serialize, TS)]
+#[ts(export, rename = "GroupBudget")]
+pub struct GroupBudgetDto {
+    group: Group,
+    #[ts(type = "number")]
+    monthly: i64,
+    /// Elle girilen haftaların toplamı aylık tutarı aşıyor
+    weekly_over: bool,
+    weeks: Vec<WeekBudgetDto>,
+}
+
+#[derive(Serialize, TS)]
+#[ts(export, rename = "WeekBudget")]
+struct WeekBudgetDto {
+    index: u32,
+    start_date: String,
+    last_date: String,
+    days: u32,
+    #[ts(type = "number")]
+    amount: i64,
+    /// Elle girildi (otomatik değil)
+    is_manual: bool,
+}
+
+impl From<GroupBudget> for GroupBudgetDto {
+    fn from(g: GroupBudget) -> Self {
+        Self {
+            group: g.group,
+            monthly: g.monthly,
+            weekly_over: g.weekly_over,
+            weeks: g
+                .weeks
+                .into_iter()
+                .map(|w| WeekBudgetDto {
+                    index: w.index,
+                    start_date: w.start_date,
+                    last_date: w.last_date,
+                    days: w.days,
+                    amount: w.amount,
+                    is_manual: w.is_manual,
+                })
+                .collect(),
+        }
+    }
+}
+
+#[derive(Deserialize, Validate, TS)]
+#[ts(export, rename = "BudgetWeekRequest")]
+pub struct WeekRequest {
+    group: Group,
+    /// Haftanın ilk günü (`PeriodWeek.start_date`)
+    week_start: String,
+    /// Kuruş; `null` → otomatiğe döndür
+    #[ts(type = "number | null")]
+    amount: Option<i64>,
+}
+
+#[derive(Deserialize, Clone, Copy, TS)]
+#[serde(rename_all = "lowercase")]
+#[ts(export, rename = "SuggestMethod")]
+pub enum Method {
+    Default,
+    Previous,
+    Avg3,
+}
+
+#[derive(Deserialize)]
+pub struct SuggestQuery {
+    method: Option<Method>,
+}
+
+#[derive(Serialize, TS)]
+#[ts(export, rename = "BudgetSuggestion")]
+pub struct SuggestionDto {
+    /// `false` → bu yöntem için veri yok (`message` nedenini söyler)
+    available: bool,
+    message: Option<String>,
+    amounts: GroupAmounts,
+}
+
+fn parse_id(id: &str) -> AppResult<Uuid> {
+    Uuid::parse_str(id).map_err(|_| AppError::NotFound)
+}
+
+pub async fn summary(
+    State(s): State<AppState>,
+    auth: AuthUser,
+    Path(id): Path<String>,
+) -> AppResult<Json<SummaryDto>> {
+    let x = budget::summary(&s.db, auth.household_id, parse_id(&id)?).await?;
+    Ok(Json(SummaryDto {
+        income_total: x.income_total,
+        fixed_expense_total: x.fixed_expense_total,
+        savings_goal: x.savings_goal,
+        carryover_debt: x.carryover_debt,
+        carryover_surplus: x.carryover_surplus,
+        spendable: x.spendable,
+        budget_total: x.budget_total,
+        unallocated: x.spendable - x.budget_total,
+    }))
+}
+
+pub async fn get(
+    State(s): State<AppState>,
+    auth: AuthUser,
+    Path(id): Path<String>,
+) -> AppResult<Json<Vec<GroupBudgetDto>>> {
+    let groups = budget::view(&s.db, auth.household_id, parse_id(&id)?).await?;
+    Ok(Json(groups.into_iter().map(GroupBudgetDto::from).collect()))
+}
+
+pub async fn set_monthly(
+    State(s): State<AppState>,
+    auth: AuthUser,
+    Path(id): Path<String>,
+    ValidJson(req): ValidJson<GroupAmounts>,
+) -> AppResult<Json<Vec<GroupBudgetDto>>> {
+    let pid = parse_id(&id)?;
+    budget::set_monthly(&s.db, auth.household_id, pid, req.into()).await?;
+    let groups = budget::view(&s.db, auth.household_id, pid).await?;
+    Ok(Json(groups.into_iter().map(GroupBudgetDto::from).collect()))
+}
+
+pub async fn set_week(
+    State(s): State<AppState>,
+    auth: AuthUser,
+    Path(id): Path<String>,
+    ValidJson(req): ValidJson<WeekRequest>,
+) -> AppResult<Json<Vec<GroupBudgetDto>>> {
+    let pid = parse_id(&id)?;
+    budget::set_week(
+        &s.db,
+        auth.household_id,
+        pid,
+        req.group,
+        &req.week_start,
+        req.amount,
+    )
+    .await?;
+    let groups = budget::view(&s.db, auth.household_id, pid).await?;
+    Ok(Json(groups.into_iter().map(GroupBudgetDto::from).collect()))
+}
+
+pub async fn suggestion(
+    State(s): State<AppState>,
+    auth: AuthUser,
+    Path(id): Path<String>,
+    Query(q): Query<SuggestQuery>,
+) -> AppResult<Json<SuggestionDto>> {
+    let method = match q.method.unwrap_or(Method::Default) {
+        Method::Default => SuggestMethod::Default,
+        Method::Previous => SuggestMethod::Previous,
+        Method::Avg3 => SuggestMethod::Avg3,
+    };
+    let x = budget::suggest(&s.db, auth.household_id, parse_id(&id)?, method).await?;
+    Ok(Json(SuggestionDto {
+        available: x.available,
+        message: x.message,
+        amounts: x.amounts.into(),
+    }))
+}
diff --git a/backend/src/routes/mod.rs b/backend/src/routes/mod.rs
index 4a735d8..587f2d8 100644
--- a/backend/src/routes/mod.rs
+++ b/backend/src/routes/mod.rs
@@ -1,6 +1,7 @@
 //! API route'larının tek giriş noktası. Tüm uç noktalar `/api` altında.
 
 mod auth;
+mod budget;
 mod catalog;
 mod health;
 mod household;
@@ -75,6 +76,13 @@ pub fn router(state: AppState) -> Router {
         )
         .route("/plan-items/{id}", patch(plan::update).delete(plan::delete))
         .route("/periods/{id}/notes", get(note::list).post(note::create))
+        .route("/periods/{id}/summary", get(budget::summary))
+        .route(
+            "/periods/{id}/budget",
+            get(budget::get).put(budget::set_monthly),
+        )
+        .route("/periods/{id}/budget/weeks", put(budget::set_week))
+        .route("/periods/{id}/budget/suggestion", get(budget::suggestion))
         .route("/notes/{id}", patch(note::update).delete(note::delete))
         // Bilinmeyen /api/... → JSON 404 (arayüzün index.html'i dönmesin)
         .fallback(|| async { AppError::NotFound });
diff --git a/backend/src/services/budget.rs b/backend/src/services/budget.rs
new file mode 100644
index 0000000..cd71388
--- /dev/null
+++ b/backend/src/services/budget.rs
@@ -0,0 +1,502 @@
+//! Bütçe ve dönem özeti (KAKEIBO_PROMPT §8.3, DATA_MODEL.md §3.8, §5).
+//!
+//! - Her grup için **aylık** tutar (`budgets`, `scope: "month"`).
+//! - Haftalık tutar saklanmaz; aylık tutar hafta gün sayısına göre bölünür.
+//!   Kullanıcı bir haftayı elle değiştirirse yalnızca o kayıt saklanır (`scope: "week"`);
+//!   kalan tutar diğer haftalara gün oranında dağıtılır → haftaların toplamı = aylık.
+//! - Özet: gelir − sabit gider − birikim − devreden borç + devreden artı = **harcanabilir**.
+
+use std::collections::HashMap;
+
+use mongodb::{
+    Database,
+    bson::{Bson, Document, Uuid, doc},
+};
+use serde::Deserialize;
+
+use crate::{
+    db::col,
+    error::{AppError, AppResult},
+    models::{
+        common::{new_meta, now, scope},
+        group::Group,
+        period::{Period, PeriodStatus},
+    },
+    services::{
+        calendar::{format_date, parse_date, weeks},
+        money::split_proportional,
+        period,
+    },
+};
+
+/// Gruplara göre tutarlar (kuruş), `Group::ALL` sırasıyla.
+pub type GroupAmounts = [i64; 4];
+
+pub struct Summary {
+    pub income_total: i64,
+    pub fixed_expense_total: i64,
+    pub savings_goal: i64,
+    pub carryover_debt: i64,
+    pub carryover_surplus: i64,
+    pub spendable: i64,
+    pub budget_total: i64,
+}
+
+pub struct WeekBudget {
+    pub index: u32,
+    pub start_date: String,
+    pub last_date: String,
+    pub days: u32,
+    pub amount: i64,
+    pub is_manual: bool,
+}
+
+pub struct GroupBudget {
+    pub group: Group,
+    pub monthly: i64,
+    pub weeks: Vec<WeekBudget>,
+    /// Elle girilen haftaların toplamı aylık tutarı aşıyor mu?
+    pub weekly_over: bool,
+}
+
+#[derive(Clone, Copy, PartialEq, Eq)]
+pub enum SuggestMethod {
+    /// Hanenin varsayılan yüzdeleri × harcanabilir tutar
+    Default,
+    /// Önceki dönemde gerçekleşen harcamalar
+    Previous,
+    /// Son 3 dönemin gerçekleşen harcama ortalaması
+    Avg3,
+}
+
+pub struct Suggestion {
+    pub available: bool,
+    pub message: Option<String>,
+    pub amounts: GroupAmounts,
+}
+
+#[derive(Deserialize)]
+struct BudgetDoc {
+    group: Group,
+    scope: String,
+    #[serde(default)]
+    week_start: Option<String>,
+    amount: i64,
+}
+
+// ---------- Özet ----------
+
+pub async fn summary(db: &Database, household_id: Uuid, period_id: Uuid) -> AppResult<Summary> {
+    let p = period::get(db, household_id, period_id).await?;
+
+    let mut income_total = 0;
+    let mut fixed_expense_total = 0;
+    for (kind, total) in sum_by(
+        db,
+        col::PLAN_ITEMS,
+        household_id,
+        doc! { "period_id": period_id },
+        "$kind",
+    )
+    .await?
+    {
+        match kind.as_str() {
+            "income" => income_total = total,
+            "expense" => fixed_expense_total = total,
+            _ => {}
+        }
+    }
+
+    let mut carryover_debt = 0;
+    let mut carryover_surplus = 0;
+    for (kind, total) in sum_by(
+        db,
+        col::CARRYOVERS,
+        household_id,
+        doc! { "to_period_id": period_id },
+        "$kind",
+    )
+    .await?
+    {
+        match kind.as_str() {
+            "debt" => carryover_debt = total,
+            "surplus" => carryover_surplus = total,
+            _ => {}
+        }
+    }
+
+    let budget_total = monthly_amounts(db, household_id, period_id)
+        .await?
+        .iter()
+        .sum();
+    let spendable =
+        income_total - fixed_expense_total - p.savings_goal - carryover_debt + carryover_surplus;
+
+    Ok(Summary {
+        income_total,
+        fixed_expense_total,
+        savings_goal: p.savings_goal,
+        carryover_debt,
+        carryover_surplus,
+        spendable,
+        budget_total,
+    })
+}
+
+// ---------- Bütçe görünümü ----------
+
+pub async fn view(
+    db: &Database,
+    household_id: Uuid,
+    period_id: Uuid,
+) -> AppResult<Vec<GroupBudget>> {
+    let p = period::get(db, household_id, period_id).await?;
+    let docs = load(db, household_id, period_id).await?;
+    let (start, end) = dates(&p)?;
+    let period_weeks = weeks(start, end);
+
+    let mut out = Vec::with_capacity(4);
+    for group in Group::ALL {
+        let monthly = docs
+            .iter()
+            .find(|d| d.group == group && d.scope == "month")
+            .map_or(0, |d| d.amount);
+        // Elle girilmiş haftalar (dönem tarihleri değiştiyse eşleşmeyen eski kayıtlar yok sayılır)
+        let manual: HashMap<String, i64> = docs
+            .iter()
+            .filter(|d| d.group == group && d.scope == "week")
+            .filter_map(|d| d.week_start.clone().map(|w| (w, d.amount)))
+            .collect();
+
+        let manual_total: i64 = period_weeks
+            .iter()
+            .filter_map(|w| manual.get(&format_date(w.start)))
+            .sum();
+        let auto_days: Vec<u64> = period_weeks
+            .iter()
+            .filter(|w| !manual.contains_key(&format_date(w.start)))
+            .map(|w| u64::from(w.days))
+            .collect();
+        let mut auto = split_proportional(monthly - manual_total, &auto_days).into_iter();
+
+        let weeks = period_weeks
+            .iter()
+            .map(|w| {
+                let key = format_date(w.start);
+                let (amount, is_manual) = match manual.get(&key) {
+                    Some(&a) => (a, true),
+                    None => (auto.next().unwrap_or(0), false),
+                };
+                WeekBudget {
+                    index: w.index,
+                    start_date: key,
+                    last_date: format_date(w.last),
+                    days: w.days,
+                    amount,
+                    is_manual,
+                }
+            })
+            .collect();
+
+        out.push(GroupBudget {
+            group,
+            monthly,
+            weeks,
+            weekly_over: manual_total > monthly,
+        });
+    }
+    Ok(out)
+}
+
+// ---------- Yazma ----------
+
+/// 4 grubun aylık tutarını birlikte kaydeder (elle girilmiş haftalar korunur).
+pub async fn set_monthly(
+    db: &Database,
+    household_id: Uuid,
+    period_id: Uuid,
+    amounts: GroupAmounts,
+) -> AppResult<()> {
+    period::get_open(db, household_id, period_id).await?;
+    for a in amounts {
+        period::validate_amount(a)?;
+    }
+    for (group, amount) in Group::ALL.into_iter().zip(amounts) {
+        upsert(db, household_id, period_id, group, "month", None, amount).await?;
+    }
+    Ok(())
+}
+
+/// Bir haftayı elle ayarlar; `amount = None` → otomatiğe döner.
+pub async fn set_week(
+    db: &Database,
+    household_id: Uuid,
+    period_id: Uuid,
+    group: Group,
+    week_start: &str,
+    amount: Option<i64>,
+) -> AppResult<()> {
+    let p = period::get_open(db, household_id, period_id).await?;
+    let (start, end) = dates(&p)?;
+    let valid =
+        parse_date(week_start).is_some_and(|d| weeks(start, end).iter().any(|w| w.start == d));
+    if !valid {
+        return Err(AppError::BadRequest("Hafta bu döneme ait değil".into()));
+    }
+    match amount {
+        Some(a) => {
+            period::validate_amount(a)?;
+            upsert(
+                db,
+                household_id,
+                period_id,
+                group,
+                "week",
+                Some(week_start),
+                a,
+            )
+            .await
+        }
+        None => {
+            let mut f = scope(household_id);
+            f.insert("period_id", period_id);
+            f.insert("group", group.as_str());
+            f.insert("scope", "week");
+            f.insert("week_start", week_start);
+            let ts = now();
+            db.collection::<Document>(col::BUDGETS)
+                .update_many(
+                    f,
+                    doc! { "$set": { "deleted_at": ts, "updated_at": ts }, "$inc": { "version": 1 } },
+                )
+                .await?;
+            Ok(())
+        }
+    }
+}
+
+// ---------- Öneri ----------
+
+pub async fn suggest(
+    db: &Database,
+    household_id: Uuid,
+    period_id: Uuid,
+    method: SuggestMethod,
+) -> AppResult<Suggestion> {
+    let p = period::get(db, household_id, period_id).await?;
+    match method {
+        SuggestMethod::Default => {
+            let s = summary(db, household_id, period_id).await?;
+            if s.spendable <= 0 {
+                return Ok(unavailable(
+                    "Harcanabilir tutar sıfır ya da negatif. Önce gelir, gider ve birikim hedefini girin.",
+                ));
+            }
+            let pct = split_percentages(db, household_id).await?;
+            let parts = split_proportional(s.spendable, &pct);
+            Ok(Suggestion {
+                available: true,
+                message: None,
+                amounts: [parts[0], parts[1], parts[2], parts[3]],
+            })
+        }
+        SuggestMethod::Previous | SuggestMethod::Avg3 => {
+            let n = if method == SuggestMethod::Previous {
+                1
+            } else {
+                3
+            };
+            let previous = previous_closed(db, household_id, &p, n).await?;
+            if previous.is_empty() {
+                return Ok(unavailable("Henüz kapanmış bir önceki dönem yok."));
+            }
+            let ids: Vec<Bson> = previous.iter().map(|id| Bson::from(*id)).collect();
+            let sums = sum_by(
+                db,
+                col::ENTRIES,
+                household_id,
+                doc! { "period_id": { "$in": ids } },
+                "$group",
+            )
+            .await?;
+            let total: i64 = sums.values().sum();
+            if total == 0 {
+                return Ok(unavailable("Önceki dönemlerde henüz harcama kaydı yok."));
+            }
+            let count = previous.len() as i64;
+            let avg = |g: Group| {
+                let s = sums.get(g.as_str()).copied().unwrap_or(0);
+                (s + count / 2) / count // yuvarlayarak bölme
+            };
+            Ok(Suggestion {
+                available: true,
+                message: None,
+                amounts: Group::ALL.map(avg),
+            })
+        }
+    }
+}
+
+// ---------- yardımcılar ----------
+
+fn dates(p: &Period) -> AppResult<(time::Date, time::Date)> {
+    let bad = || AppError::Internal("dönem tarihi okunamadı".into());
+    Ok((
+        parse_date(&p.start_date).ok_or_else(bad)?,
+        parse_date(&p.end_date).ok_or_else(bad)?,
+    ))
+}
+
+fn unavailable(msg: &str) -> Suggestion {
+    Suggestion {
+        available: false,
+        message: Some(msg.into()),
+        amounts: [0; 4],
+    }
+}
+
+async fn load(db: &Database, household_id: Uuid, period_id: Uuid) -> AppResult<Vec<BudgetDoc>> {
+    let mut f = scope(household_id);
+    f.insert("period_id", period_id);
+    let mut cursor = db.collection::<BudgetDoc>(col::BUDGETS).find(f).await?;
+    let mut out = Vec::new();
+    while cursor.advance().await? {
+        out.push(cursor.deserialize_current()?);
+    }
+    Ok(out)
+}
+
+async fn monthly_amounts(
+    db: &Database,
+    household_id: Uuid,
+    period_id: Uuid,
+) -> AppResult<GroupAmounts> {
+    let docs = load(db, household_id, period_id).await?;
+    Ok(Group::ALL.map(|g| {
+        docs.iter()
+            .find(|d| d.group == g && d.scope == "month")
+            .map_or(0, |d| d.amount)
+    }))
+}
+
+async fn upsert(
+    db: &Database,
+    household_id: Uuid,
+    period_id: Uuid,
+    group: Group,
+    level: &str,
+    week_start: Option<&str>,
+    amount: i64,
+) -> AppResult<()> {
+    let coll = db.collection::<Document>(col::BUDGETS);
+    let mut f = scope(household_id);
+    f.insert("period_id", period_id);
+    f.insert("group", group.as_str());
+    f.insert("scope", level);
+    f.insert(
+        "week_start",
+        week_start.map_or(Bson::Null, |w| Bson::String(w.into())),
+    );
+
+    let res = coll
+        .update_one(
+            f,
+            doc! { "$set": { "amount": amount, "updated_at": now() }, "$inc": { "version": 1 } },
+        )
+        .await?;
+    if res.matched_count == 0 {
+        let mut d = new_meta(household_id);
+        d.insert("period_id", period_id);
+        d.insert("group", group.as_str());
+        d.insert("scope", level);
+        d.insert(
+            "week_start",
+            week_start.map_or(Bson::Null, |w| Bson::String(w.into())),
+        );
+        d.insert("amount", amount);
+        coll.insert_one(d).await?;
+    }
+    Ok(())
+}
+
+/// Bir koleksiyonda `amount` alanını bir alana göre toplar: `{ "income": 450000, ... }`
+async fn sum_by(
+    db: &Database,
+    collection: &str,
+    household_id: Uuid,
+    filter: Document,
+    key: &str,
+) -> AppResult<HashMap<String, i64>> {
+    let mut m = scope(household_id);
+    m.extend(filter);
+    let pipeline = vec![
+        doc! { "$match": m },
+        doc! { "$group": { "_id": key, "total": { "$sum": "$amount" } } },
+    ];
+    let mut cursor = db
+        .collection::<Document>(collection)
+        .aggregate(pipeline)
+        .await?;
+    let mut out = HashMap::new();
+    while cursor.advance().await? {
+        let d = cursor.deserialize_current()?;
+        let k = d.get_str("_id").unwrap_or_default().to_string();
+        let v = match d.get("total") {
+            Some(Bson::Int64(v)) => *v,
+            Some(Bson::Int32(v)) => i64::from(*v),
+            _ => 0,
+        };
+        out.insert(k, v);
+    }
+    Ok(out)
+}
+
+/// Hane ayarındaki varsayılan yüzdeler (yoksa 50/30/10/10).
+async fn split_percentages(db: &Database, household_id: Uuid) -> AppResult<Vec<u64>> {
+    let mut f = scope(household_id);
+    f.insert("_id", household_id);
+    let hh = db
+        .collection::<Document>(col::HOUSEHOLDS)
+        .find_one(f)
+        .await?;
+    let pct = hh
+        .as_ref()
+        .and_then(|h| h.get_document("settings").ok())
+        .and_then(|s| s.get_document("budget_split_pct").ok());
+    let default = [50u64, 30, 10, 10];
+    Ok(Group::ALL
+        .iter()
+        .zip(default)
+        .map(|(g, d)| {
+            pct.and_then(|p| match p.get(g.as_str()) {
+                Some(Bson::Int32(v)) => Some(*v as u64),
+                Some(Bson::Int64(v)) => Some(*v as u64),
+                _ => None,
+            })
+            .unwrap_or(d)
+        })
+        .collect())
+}
+
+/// Bu dönemden önceki en yeni `n` kapalı dönemin kimlikleri.
+async fn previous_closed(
+    db: &Database,
+    household_id: Uuid,
+    p: &Period,
+    n: i64,
+) -> AppResult<Vec<Uuid>> {
+    let mut f = scope(household_id);
+    f.insert("start_date", doc! { "$lt": &p.start_date });
+    f.insert("status", PeriodStatus::Closed.as_str());
+    let mut cursor = db
+        .collection::<Period>(col::PERIODS)
+        .find(f)
+        .sort(doc! { "start_date": -1 })
+        .limit(n)
+        .await?;
+    let mut out = Vec::new();
+    while cursor.advance().await? {
+        out.push(cursor.deserialize_current()?.id);
+    }
+    Ok(out)
+}
diff --git a/backend/src/services/mod.rs b/backend/src/services/mod.rs
index 0a972c5..44a7084 100644
--- a/backend/src/services/mod.rs
+++ b/backend/src/services/mod.rs
@@ -1,9 +1,11 @@
 //! İş mantığı. Route'lar ince kalır; asıl işi buradaki fonksiyonlar yapar.
 
 pub mod account;
+pub mod budget;
 pub mod calendar;
 pub mod catalog;
 pub mod invite;
+pub mod money;
 pub mod note;
 pub mod period;
 pub mod plan;
diff --git a/backend/src/services/money.rs b/backend/src/services/money.rs
new file mode 100644
index 0000000..67fceec
--- /dev/null
+++ b/backend/src/services/money.rs
@@ -0,0 +1,68 @@
+//! Para hesapları (kuruş, `i64`). Float yok.
+
+/// `total` kuruşu ağırlıklara orantılı böler; **toplam her zaman `total`'a eşittir**.
+///
+/// "En büyük kalan" yöntemi: herkes tam payını alır, artan kuruşlar
+/// küsuratı en büyük olana (eşitlikte öndekine) birer birer verilir.
+/// Ağırlıkların hepsi 0 ise hepsine 0 döner (ve `total` dağıtılamaz → çağıran kontrol eder).
+pub fn split_proportional(total: i64, weights: &[u64]) -> Vec<i64> {
+    let sum: u128 = weights.iter().map(|&w| u128::from(w)).sum();
+    if sum == 0 || total <= 0 {
+        return vec![0; weights.len()];
+    }
+    let total_u = total as u128;
+    let mut parts: Vec<i64> = Vec::with_capacity(weights.len());
+    let mut remainders: Vec<(u128, usize)> = Vec::with_capacity(weights.len());
+    for (i, &w) in weights.iter().enumerate() {
+        let exact = total_u * u128::from(w);
+        parts.push((exact / sum) as i64);
+        remainders.push((exact % sum, i));
+    }
+    let mut left = total - parts.iter().sum::<i64>();
+    // Büyük kalan önce; eşitlikte küçük indeks önce
+    remainders.sort_by(|a, b| b.0.cmp(&a.0).then(a.1.cmp(&b.1)));
+    for (_, i) in remainders {
+        if left == 0 {
+            break;
+        }
+        parts[i] += 1;
+        left -= 1;
+    }
+    parts
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn sums_exactly_and_is_proportional() {
+        // 1.000,00 ₺ Aralık haftalarına (6,7,7,7,4 gün)
+        let p = split_proportional(100_000, &[6, 7, 7, 7, 4]);
+        assert_eq!(p.iter().sum::<i64>(), 100_000);
+        assert_eq!(p, vec![19_355, 22_581, 22_581, 22_580, 12_903]);
+    }
+
+    #[test]
+    fn default_percentages() {
+        // Tam paylar 617.283,5 / 370.370,1 / 123.456,7 / 123.456,7 → artan 2 kuruş
+        // küsuratı en büyük olan ikisine (0,7'ler) gider
+        assert_eq!(
+            split_proportional(1_234_567, &[50, 30, 10, 10]),
+            vec![617_283, 370_370, 123_457, 123_457]
+        );
+        assert_eq!(split_proportional(1, &[50, 30, 10, 10]), vec![1, 0, 0, 0]);
+    }
+
+    #[test]
+    fn edge_cases() {
+        assert_eq!(split_proportional(0, &[1, 2]), vec![0, 0]);
+        assert_eq!(split_proportional(-500, &[1, 2]), vec![0, 0]);
+        assert_eq!(split_proportional(100, &[0, 0]), vec![0, 0]);
+        assert_eq!(split_proportional(100, &[]), Vec::<i64>::new());
+        assert_eq!(split_proportional(7, &[0, 3, 0]), vec![0, 7, 0]);
+        // Çok büyük tutarlarda taşma yok
+        let big = 10_000_000_000_000;
+        assert_eq!(split_proportional(big, &[31, 31]).iter().sum::<i64>(), big);
+    }
+}
diff --git a/backend/tests/budget.rs b/backend/tests/budget.rs
new file mode 100644
index 0000000..f1254f4
--- /dev/null
+++ b/backend/tests/budget.rs
@@ -0,0 +1,403 @@
+//! Bütçe ve dönem özeti.
+
+mod common;
+
+use axum::http::StatusCode;
+use common::{Session, TestApp};
+use mongodb::bson::{Document, Uuid, doc};
+use serde_json::{Value, json};
+
+async fn open_december(app: &TestApp, s: &Session, savings_goal: i64) -> String {
+    let (st, p) = app
+        .post(
+            "/periods",
+            s,
+            json!({ "start_date": "2026-12-01", "end_date": "2027-01-01", "savings_goal": savings_goal }),
+        )
+        .await;
+    assert_eq!(st, StatusCode::CREATED, "{p}");
+    p["id"].as_str().unwrap().to_string()
+}
+
+async fn add_item(app: &TestApp, s: &Session, pid: &str, kind: &str, amount: i64) {
+    let (st, b) = app
+        .post(
+            &format!("/periods/{pid}/plan-items"),
+            s,
+            json!({ "kind": kind, "name": format!("{kind}-{amount}"), "amount": amount }),
+        )
+        .await;
+    assert_eq!(st, StatusCode::CREATED, "{b}");
+}
+
+fn group<'a>(budget: &'a Value, code: &str) -> &'a Value {
+    budget
+        .as_array()
+        .unwrap()
+        .iter()
+        .find(|g| g["group"] == code)
+        .unwrap()
+}
+
+fn week_amounts(g: &Value) -> Vec<i64> {
+    g["weeks"]
+        .as_array()
+        .unwrap()
+        .iter()
+        .map(|w| w["amount"].as_i64().unwrap())
+        .collect()
+}
+
+#[tokio::test]
+async fn summary_computes_spendable_with_carryovers() {
+    let app = TestApp::new().await;
+    let s = app.register("Test", None).await;
+    let pid = open_december(&app, &s, 500_000).await;
+
+    // Boş dönem: gelir yok → harcanabilir = -birikim
+    let (_, sum) = app.get(&format!("/periods/{pid}/summary"), &s).await;
+    assert_eq!(sum["spendable"], -500_000);
+
+    add_item(&app, &s, &pid, "income", 4_500_000).await;
+    add_item(&app, &s, &pid, "income", 300_000).await;
+    add_item(&app, &s, &pid, "expense", 1_500_000).await;
+    add_item(&app, &s, &pid, "expense", 45_000).await;
+
+    // Önceki dönemden devir (Faz 6'da kapanış sihirbazı yazacak)
+    let hid = Uuid::parse_str(&s.household_id).unwrap();
+    let to = Uuid::parse_str(&pid).unwrap();
+    app.db
+        .collection::<Document>("carryovers")
+        .insert_many([
+            doc! { "household_id": hid, "to_period_id": to, "kind": "debt", "amount": 100_000_i64, "deleted_at": null },
+            doc! { "household_id": hid, "to_period_id": to, "kind": "surplus", "amount": 20_000_i64, "deleted_at": null },
+        ])
+        .await
+        .unwrap();
+
+    let (st, sum) = app.get(&format!("/periods/{pid}/summary"), &s).await;
+    assert_eq!(st, StatusCode::OK);
+    assert_eq!(
+        sum,
+        json!({
+            "income_total": 4_800_000,
+            "fixed_expense_total": 1_545_000,
+            "savings_goal": 500_000,
+            "carryover_debt": 100_000,
+            "carryover_surplus": 20_000,
+            // 4.800.000 − 1.545.000 − 500.000 − 100.000 + 20.000
+            "spendable": 2_675_000,
+            "budget_total": 0,
+            "unallocated": 2_675_000
+        })
+    );
+
+    app.cleanup().await;
+}
+
+#[tokio::test]
+async fn default_suggestion_splits_spendable() {
+    let app = TestApp::new().await;
+    let s = app.register("Test", None).await;
+    let pid = open_december(&app, &s, 0).await;
+    let url = format!("/periods/{pid}/budget/suggestion?method=default");
+
+    // Gelir yokken öneri yok
+    let (_, sug) = app.get(&url, &s).await;
+    assert_eq!(sug["available"], false);
+    assert!(sug["message"].as_str().unwrap().contains("Harcanabilir"));
+
+    add_item(&app, &s, &pid, "income", 1_234_567).await;
+    let (_, sug) = app.get(&url, &s).await;
+    assert_eq!(sug["available"], true);
+    assert_eq!(
+        sug["amounts"],
+        json!({ "needs": 617_283, "wants": 370_370, "culture": 123_457, "unexpected": 123_457 })
+    );
+
+    // Önceki dönem yokken previous/avg3 → veri yok
+    for m in ["previous", "avg3"] {
+        let (_, sug) = app
+            .get(&format!("/periods/{pid}/budget/suggestion?method={m}"), &s)
+            .await;
+        assert_eq!(sug["available"], false, "{m}");
+    }
+
+    app.cleanup().await;
+}
+
+#[tokio::test]
+async fn monthly_budget_is_split_into_weeks_by_days() {
+    let app = TestApp::new().await;
+    let s = app.register("Test", None).await;
+    let pid = open_december(&app, &s, 0).await;
+    add_item(&app, &s, &pid, "income", 300_000).await;
+    let url = format!("/periods/{pid}/budget");
+
+    // Başta hepsi 0
+    let (_, b) = app.get(&url, &s).await;
+    assert_eq!(b.as_array().unwrap().len(), 4);
+    assert_eq!(week_amounts(group(&b, "needs")), vec![0; 5]);
+
+    let (st, b) = app
+        .put(
+            &url,
+            &s,
+            json!({ "needs": 100_000, "wants": 50_000, "culture": 31, "unexpected": 0 }),
+        )
+        .await;
+    assert_eq!(st, StatusCode::OK, "{b}");
+    let needs = group(&b, "needs");
+    assert_eq!(needs["monthly"], 100_000);
+    // Aralık: 6,7,7,7,4 gün → toplam tam 100.000
+    assert_eq!(
+        week_amounts(needs),
+        vec![19_355, 22_581, 22_581, 22_580, 12_903]
+    );
+    assert_eq!(week_amounts(group(&b, "culture")), vec![6, 7, 7, 7, 4]);
+    for g in b.as_array().unwrap() {
+        assert_eq!(
+            week_amounts(g).iter().sum::<i64>(),
+            g["monthly"].as_i64().unwrap()
+        );
+    }
+
+    // Özet: 150.031 dağıtıldı, harcanabilir 300.000 → kalan 149.969
+    let (_, sum) = app.get(&format!("/periods/{pid}/summary"), &s).await;
+    assert_eq!(
+        (sum["budget_total"].as_i64(), sum["unallocated"].as_i64()),
+        (Some(150_031), Some(149_969))
+    );
+
+    // Fazla dağıtım → kalan negatif (arayüz uyarı gösterir)
+    app.put(
+        &url,
+        &s,
+        json!({ "needs": 400_000, "wants": 0, "culture": 0, "unexpected": 0 }),
+    )
+    .await;
+    let (_, sum) = app.get(&format!("/periods/{pid}/summary"), &s).await;
+    assert_eq!(sum["unallocated"], -100_000);
+
+    // Negatif tutar → 400
+    let (st, _) = app
+        .put(
+            &url,
+            &s,
+            json!({ "needs": -1, "wants": 0, "culture": 0, "unexpected": 0 }),
+        )
+        .await;
+    assert_eq!(st, StatusCode::BAD_REQUEST);
+
+    app.cleanup().await;
+}
+
+#[tokio::test]
+async fn manual_week_override_redistributes_the_rest() {
+    let app = TestApp::new().await;
+    let s = app.register("Test", None).await;
+    let pid = open_december(&app, &s, 0).await;
+    app.put(
+        &format!("/periods/{pid}/budget"),
+        &s,
+        json!({ "needs": 100_000, "wants": 0, "culture": 0, "unexpected": 0 }),
+    )
+    .await;
+    let weeks_url = format!("/periods/{pid}/budget/weeks");
+
+    // 2. hafta (7–13 Ara) elle 50.000 → kalan 50.000 diğer haftalara (6+7+7+4 = 24 gün)
+    let (st, b) = app
+        .put(
+            &weeks_url,
+            &s,
+            json!({ "group": "needs", "week_start": "2026-12-07", "amount": 50_000 }),
+        )
+        .await;
+    assert_eq!(st, StatusCode::OK, "{b}");
+    let needs = group(&b, "needs");
+    assert_eq!(
+        week_amounts(needs),
+        vec![12_500, 50_000, 14_584, 14_583, 8_333]
+    );
+    assert_eq!(week_amounts(needs).iter().sum::<i64>(), 100_000);
+    let manual: Vec<bool> = needs["weeks"]
+        .as_array()
+        .unwrap()
+        .iter()
+        .map(|w| w["is_manual"].as_bool().unwrap())
+        .collect();
+    assert_eq!(manual, vec![false, true, false, false, false]);
+    assert_eq!(needs["weekly_over"], false);
+
+    // Aylık tutar değişince elle girilen hafta korunur
+    let (_, b) = app
+        .put(
+            &format!("/periods/{pid}/budget"),
+            &s,
+            json!({ "needs": 74_000, "wants": 0, "culture": 0, "unexpected": 0 }),
+        )
+        .await;
+    assert_eq!(week_amounts(group(&b, "needs"))[1], 50_000);
+    assert_eq!(week_amounts(group(&b, "needs")).iter().sum::<i64>(), 74_000);
+
+    // Elle girilenler aylığı aşarsa uyarı; diğer haftalar 0
+    let (_, b) = app
+        .put(
+            &weeks_url,
+            &s,
+            json!({ "group": "needs", "week_start": "2026-12-14", "amount": 30_000 }),
+        )
+        .await;
+    let needs = group(&b, "needs");
+    assert_eq!(needs["weekly_over"], true);
+    assert_eq!(week_amounts(needs), vec![0, 50_000, 30_000, 0, 0]);
+
+    // null → otomatiğe dön
+    app.put(
+        &weeks_url,
+        &s,
+        json!({ "group": "needs", "week_start": "2026-12-14", "amount": null }),
+    )
+    .await;
+    let (_, b) = app
+        .put(
+            &weeks_url,
+            &s,
+            json!({ "group": "needs", "week_start": "2026-12-07", "amount": null }),
+        )
+        .await;
+    assert_eq!(week_amounts(group(&b, "needs")).iter().sum::<i64>(), 74_000);
+    assert!(
+        group(&b, "needs")["weeks"]
+            .as_array()
+            .unwrap()
+            .iter()
+            .all(|w| w["is_manual"] == false)
+    );
+
+    // Haftanın ilk günü olmayan tarih → 400
+    let (st, _) = app
+        .put(
+            &weeks_url,
+            &s,
+            json!({ "group": "needs", "week_start": "2026-12-08", "amount": 1 }),
+        )
+        .await;
+    assert_eq!(st, StatusCode::BAD_REQUEST);
+
+    app.cleanup().await;
+}
+
+#[tokio::test]
+async fn previous_and_avg3_suggestions_use_actual_spending() {
+    let app = TestApp::new().await;
+    let s = app.register("Test", None).await;
+    let hid = Uuid::parse_str(&s.household_id).unwrap();
+
+    // Ekim ve Kasım kapalı, harcamaları var (Faz 5'te defterden gelecek)
+    let mut closed = Vec::new();
+    for (start, end) in [("2026-10-01", "2026-11-01"), ("2026-11-01", "2026-12-01")] {
+        let (_, p) = app
+            .post(
+                "/periods",
+                &s,
+                json!({ "start_date": start, "end_date": end }),
+            )
+            .await;
+        let id = Uuid::parse_str(p["id"].as_str().unwrap()).unwrap();
+        app.db
+            .collection::<Document>("periods")
+            .update_one(doc! { "_id": id }, doc! { "$set": { "status": "closed" } })
+            .await
+            .unwrap();
+        closed.push(id);
+    }
+    let entry = |pid: Uuid, group: &str, amount: i64| {
+        doc! { "household_id": hid, "period_id": pid, "group": group, "amount": amount, "date": "2026-10-10", "deleted_at": null }
+    };
+    app.db
+        .collection::<Document>("entries")
+        .insert_many([
+            entry(closed[0], "needs", 100_000),
+            entry(closed[0], "wants", 41_000),
+            entry(closed[1], "needs", 200_000),
+            entry(closed[1], "needs", 50_000),
+            entry(closed[1], "culture", 10_000),
+        ])
+        .await
+        .unwrap();
+
+    let pid = open_december(&app, &s, 0).await;
+    let (_, prev) = app
+        .get(
+            &format!("/periods/{pid}/budget/suggestion?method=previous"),
+            &s,
+        )
+        .await;
+    assert_eq!(prev["available"], true, "{prev}");
+    assert_eq!(
+        prev["amounts"],
+        json!({ "needs": 250_000, "wants": 0, "culture": 10_000, "unexpected": 0 })
+    );
+
+    // avg3: 2 kapalı dönem var → ikiye bölünür (yuvarlayarak)
+    let (_, avg) = app
+        .get(&format!("/periods/{pid}/budget/suggestion?method=avg3"), &s)
+        .await;
+    assert_eq!(
+        avg["amounts"],
+        json!({ "needs": 175_000, "wants": 20_500, "culture": 5_000, "unexpected": 0 })
+    );
+
+    app.cleanup().await;
+}
+
+#[tokio::test]
+async fn budget_closed_period_and_isolation() {
+    let app = TestApp::new().await;
+    let a = app.register("A", None).await;
+    let b = app.register("B", None).await;
+    let pid = open_december(&app, &a, 0).await;
+    let body = json!({ "needs": 1, "wants": 0, "culture": 0, "unexpected": 0 });
+
+    // B erişemez
+    let checks = [
+        app.get(&format!("/periods/{pid}/summary"), &b).await.0,
+        app.get(&format!("/periods/{pid}/budget"), &b).await.0,
+        app.put(&format!("/periods/{pid}/budget"), &b, body.clone())
+            .await
+            .0,
+        app.put(
+            &format!("/periods/{pid}/budget/weeks"),
+            &b,
+            json!({ "group": "needs", "week_start": "2026-12-07", "amount": 1 }),
+        )
+        .await
+        .0,
+        app.get(&format!("/periods/{pid}/budget/suggestion"), &b)
+            .await
+            .0,
+    ];
+    assert!(
+        checks.iter().all(|s| *s == StatusCode::NOT_FOUND),
+        "{checks:?}"
+    );
+
+    // Kapalı dönem: okuma serbest, yazma 409
+    let id = Uuid::parse_str(&pid).unwrap();
+    app.db
+        .collection::<Document>("periods")
+        .update_one(doc! { "_id": id }, doc! { "$set": { "status": "closed" } })
+        .await
+        .unwrap();
+    assert_eq!(
+        app.get(&format!("/periods/{pid}/budget"), &a).await.0,
+        StatusCode::OK
+    );
+    assert_eq!(
+        app.put(&format!("/periods/{pid}/budget"), &a, body).await.0,
+        StatusCode::CONFLICT
+    );
+
+    app.cleanup().await;
+}
diff --git a/docs/MEMORY.md b/docs/MEMORY.md
index 276009f..ec3375e 100644
--- a/docs/MEMORY.md
+++ b/docs/MEMORY.md
@@ -1,7 +1,7 @@
 # 🧠 Proje Hafızası
 
 ## ▶️ Sıradaki Adım
-**Faz 4 – Adım 3:** Backend bütçe: `budgets` (dönem × grup aylık tutar; haftalık yalnızca elle değiştirilirse `scope: week`), haftalık otomatik = aylık × hafta günü / dönem günü (kuruş yuvarlama: toplam aylığa eşit kalsın), öneri (`default` yüzdeler; `previous`/`avg3` entries gerektirir → şimdilik boş/varsayılana düşer), özet endpoint'i: gelir − sabit gider − birikim − devreden borç = harcanabilir; dağıtılan > harcanabilir uyarısı.
+**Faz 4 – Adım 4:** Frontend ilk dönem sihirbazı: açık dönem yoksa (Panel/Plan) → adımlar: 1) tarihler (varsayılan: içinde bulunulan ay, ya da önceki dönemin bitişi), 2) sabit gelirler, 3) sabit giderler (+ önceki dönemden kopyala varsa), 4) birikim hedefi, 5) bütçe dağıtımı (öneriyle) + özet. `lib/format.ts` parseMoney ile tutar girişi (inputMode=decimal). TanStack Query anahtarları: period/current, plan-items, summary, budget.
 
 ## 📌 Alınan Kararlar
 - **Yerel demo hesapları** (`kakeibo_dev`): `demo@ornek.com` / `demo12345` (owner, "Demo Hanesi") · `es@ornek.com` / `demo12345` (member, aynı hane). `npm run db:reset` silerse API ile yeniden oluştur (ileride `db:seed`).
@@ -40,6 +40,9 @@
 - ts-rs: `i64` tutarlar `#[ts(type = "number")]`; isteklerde `Option<i64>` için `#[ts(type = "number", optional)]` (yoksa `?` kayboluyor).
 - Plan kalemleri: tarih isteğe bağlı (dönem içinde), tutar > 0, kopyalama göreli gün + dönem sonuna kırpma, aynı tür+ad (büyük/küçük harf duyarsız) atlanır. Notlar: başlık 1–120, metin ≤ 20.000. Kapalı dönemde hepsi 409 (`period::get_open`). Silme = soft delete.
 - İzolasyon iki katmanlı: kayıt okuma `scope()` + dönem `get_open(hid, period_id)` + yazma filtresi `scope()`. Tek katmanı bozmak açık oluşturmuyor (mutasyon denemesiyle görüldü).
+- Bütçe: aylık = `budgets{scope:"month"}`; haftalık otomatik = (aylık − elle girilenler) gün oranında, `money::split_proportional` (en büyük kalan, toplam tam eşit). Elle hafta = `scope:"week", week_start`; `null` → soft delete. Aylık değişince elle haftalar korunur; elle toplam > aylık → `weekly_over`.
+- Özet: spendable = gelir − sabit gider − birikim − borç + artı (`carryovers.to_period_id`). `unallocated` = spendable − bütçe toplamı (negatif = fazla dağıtım).
+- Öneri: default = hane `settings.budget_split_pct` × spendable (≤0 → available:false); previous/avg3 = önceki kapalı dönem(ler)in `entries` toplamı / dönem sayısı (yuvarlayarak).
 - Sürümler: Rust 1.99.1 (edition 2024), Axum 0.8, React 19, Vite 8, TypeScript 7, Tailwind 4.3, shadcn 4.23, Vitest 5, react-router 8, TanStack Query 5.
 
 ## ❓ Açık Konular
@@ -87,3 +90,4 @@
 ### 2026-11-12 — Faz 4
 - **Adım 1:** `services/calendar.rs` (parse/format, `weeks` Pazartesi başlangıçlı + kısmi haftalar, Türkçe ay adları; 4 test), `models/period.rs`, `services/period.rs` (2 test), `routes/period.rs` (list/current/create/get/patch; DTO'da `last_date`, `days`, `weeks`). `tests/periods.rs` (5) — çakışma kontrolü bilerek bozulunca test yakaladı. Backend toplam 58 test.
 - **Adım 2:** `models/plan_item.rs`, `models/note.rs`, `services/plan.rs` (list/create/update/delete/copy_previous), `services/note.rs`, `routes/plan.rs`, `routes/note.rs`, `period::get_open` + `closed()`. `tests/plan_notes.rs` (5): CRUD + doğrulama, kopyalama (göreli gün, 28 Şubat'a kırpma, tekrar yok), kapalı dönem salt okunur, not filtresi, izolasyon. Backend 72 test.
+- **Adım 3:** `services/money.rs` (`split_proportional` + 3 test; beklenen değer hatası testteydi, elle hesapla düzeltildi), `services/budget.rs` (summary, view, set_monthly, set_week, suggest), `routes/budget.rs` (5 uç nokta, 9 TS tipi). `tests/budget.rs` (6) — özet formülü bilerek bozulunca yakaladı. Backend 88 test.
diff --git a/docs/PHASES.md b/docs/PHASES.md
index 21f9d1c..acfe183 100644
--- a/docs/PHASES.md
+++ b/docs/PHASES.md
@@ -48,7 +48,7 @@
 ## Faz 4 Adımları
 - ✅ 1. Backend: dönem (aç, açık dönem, liste, detay + haftalar, düzenle), takvim hesapları, testler
 - ✅ 2. Backend: plan kalemleri (sabit gelir/gider, önceki dönemden kopyala) + notlar
-- 🔄 3. Backend: bütçe (aylık/haftalık, öneri), özet (harcanabilir tutar)
-- ⏳ 4. Frontend: ilk dönem sihirbazı
+- ✅ 3. Backend: bütçe (aylık/haftalık, öneri), özet (harcanabilir tutar)
+- 🔄 4. Frontend: ilk dönem sihirbazı
 - ⏳ 5. Frontend: Plan ekranı (görüntüle/düzenle, özet kutucukları, bütçe dağıtımı)
 - ⏳ 6. Frontend: notlar (Markdown, düzenle/önizle)
diff --git a/frontend/src/types/BudgetSuggestion.ts b/frontend/src/types/BudgetSuggestion.ts
new file mode 100644
index 0000000..f623871
--- /dev/null
+++ b/frontend/src/types/BudgetSuggestion.ts
@@ -0,0 +1,8 @@
+// This file was generated by [ts-rs](https://github.com/Aleph-Alpha/ts-rs). Do not edit this file manually.
+import type { GroupAmounts } from "./GroupAmounts";
+
+export type BudgetSuggestion = { 
+/**
+ * `false` → bu yöntem için veri yok (`message` nedenini söyler)
+ */
+available: boolean, message: string | null, amounts: GroupAmounts, };
diff --git a/frontend/src/types/BudgetWeekRequest.ts b/frontend/src/types/BudgetWeekRequest.ts
new file mode 100644
index 0000000..08607b3
--- /dev/null
+++ b/frontend/src/types/BudgetWeekRequest.ts
@@ -0,0 +1,12 @@
+// This file was generated by [ts-rs](https://github.com/Aleph-Alpha/ts-rs). Do not edit this file manually.
+import type { Group } from "./Group";
+
+export type BudgetWeekRequest = { group: Group, 
+/**
+ * Haftanın ilk günü (`PeriodWeek.start_date`)
+ */
+week_start: string, 
+/**
+ * Kuruş; `null` → otomatiğe döndür
+ */
+amount: number | null, };
diff --git a/frontend/src/types/GroupAmounts.ts b/frontend/src/types/GroupAmounts.ts
new file mode 100644
index 0000000..5b85198
--- /dev/null
+++ b/frontend/src/types/GroupAmounts.ts
@@ -0,0 +1,6 @@
+// This file was generated by [ts-rs](https://github.com/Aleph-Alpha/ts-rs). Do not edit this file manually.
+
+/**
+ * Gruplara göre tutarlar (kuruş)
+ */
+export type GroupAmounts = { needs: number, wants: number, culture: number, unexpected: number, };
diff --git a/frontend/src/types/GroupBudget.ts b/frontend/src/types/GroupBudget.ts
new file mode 100644
index 0000000..0214404
--- /dev/null
+++ b/frontend/src/types/GroupBudget.ts
@@ -0,0 +1,9 @@
+// This file was generated by [ts-rs](https://github.com/Aleph-Alpha/ts-rs). Do not edit this file manually.
+import type { Group } from "./Group";
+import type { WeekBudget } from "./WeekBudget";
+
+export type GroupBudget = { group: Group, monthly: number, 
+/**
+ * Elle girilen haftaların toplamı aylık tutarı aşıyor
+ */
+weekly_over: boolean, weeks: Array<WeekBudget>, };
diff --git a/frontend/src/types/PeriodSummary.ts b/frontend/src/types/PeriodSummary.ts
new file mode 100644
index 0000000..301054e
--- /dev/null
+++ b/frontend/src/types/PeriodSummary.ts
@@ -0,0 +1,23 @@
+// This file was generated by [ts-rs](https://github.com/Aleph-Alpha/ts-rs). Do not edit this file manually.
+
+export type PeriodSummary = { income_total: number, fixed_expense_total: number, savings_goal: number, 
+/**
+ * Önceki dönemden devreden borç
+ */
+carryover_debt: number, 
+/**
+ * Önceki dönemden devreden artı bakiye
+ */
+carryover_surplus: number, 
+/**
+ * gelir − sabit gider − birikim − borç + artı
+ */
+spendable: number, 
+/**
+ * 4 grubun aylık bütçe toplamı
+ */
+budget_total: number, 
+/**
+ * harcanabilir − dağıtılan (negatifse fazla dağıtılmış)
+ */
+unallocated: number, };
diff --git a/frontend/src/types/SuggestMethod.ts b/frontend/src/types/SuggestMethod.ts
new file mode 100644
index 0000000..5f04091
--- /dev/null
+++ b/frontend/src/types/SuggestMethod.ts
@@ -0,0 +1,3 @@
+// This file was generated by [ts-rs](https://github.com/Aleph-Alpha/ts-rs). Do not edit this file manually.
+
+export type SuggestMethod = "default" | "previous" | "avg3";
diff --git a/frontend/src/types/WeekBudget.ts b/frontend/src/types/WeekBudget.ts
new file mode 100644
index 0000000..9fe5e0a
--- /dev/null
+++ b/frontend/src/types/WeekBudget.ts
@@ -0,0 +1,7 @@
+// This file was generated by [ts-rs](https://github.com/Aleph-Alpha/ts-rs). Do not edit this file manually.
+
+export type WeekBudget = { index: number, start_date: string, last_date: string, days: number, amount: number, 
+/**
+ * Elle girildi (otomatik değil)
+ */
+is_manual: boolean, };