diff --git a/backend/app/services/rebalance.py b/backend/app/services/rebalance.py
new file mode 100644
index 0000000..d79f83a
--- /dev/null
+++ b/backend/app/services/rebalance.py
@@ -0,0 +1,154 @@
+"""
+Rebalancing calculation service.
+"""
+from decimal import Decimal
+from typing import List, Dict, Optional
+
+from sqlalchemy.orm import Session
+
+from app.models.portfolio import Portfolio
+from app.models.stock import Stock, ETF, ETFPrice
+from app.schemas.portfolio import RebalanceItem, RebalanceResponse, RebalanceSimulationResponse
+
+
+class RebalanceService:
+    """Service for calculating portfolio rebalancing."""
+
+    def __init__(self, db: Session):
+        self.db = db
+
+    def get_current_prices(self, tickers: List[str]) -> Dict[str, Decimal]:
+        """Get current prices for tickers from database."""
+        prices = {}
+
+        # Check stocks
+        stocks = self.db.query(Stock).filter(Stock.ticker.in_(tickers)).all()
+        for stock in stocks:
+            if stock.close_price:
+                prices[stock.ticker] = Decimal(str(stock.close_price))
+
+        # Check ETFs for missing tickers
+        missing = [t for t in tickers if t not in prices]
+        if missing:
+            # Get latest ETF prices
+            from sqlalchemy import func
+            subq = (
+                self.db.query(
+                    ETFPrice.ticker,
+                    func.max(ETFPrice.date).label('max_date')
+                )
+                .filter(ETFPrice.ticker.in_(missing))
+                .group_by(ETFPrice.ticker)
+                .subquery()
+            )
+            etf_prices = (
+                self.db.query(ETFPrice)
+                .join(subq, (ETFPrice.ticker == subq.c.ticker) & (ETFPrice.date == subq.c.max_date))
+                .all()
+            )
+            for ep in etf_prices:
+                prices[ep.ticker] = Decimal(str(ep.close))
+
+        return prices
+
+    def get_stock_names(self, tickers: List[str]) -> Dict[str, str]:
+        """Get stock names for tickers."""
+        names = {}
+        stocks = self.db.query(Stock).filter(Stock.ticker.in_(tickers)).all()
+        for stock in stocks:
+            names[stock.ticker] = stock.name
+
+        # Also check ETFs
+        missing = [t for t in tickers if t not in names]
+        if missing:
+            etfs = self.db.query(ETF).filter(ETF.ticker.in_(missing)).all()
+            for etf in etfs:
+                names[etf.ticker] = etf.name
+
+        return names
+
+    def calculate_rebalance(
+        self,
+        portfolio: Portfolio,
+        additional_amount: Optional[Decimal] = None,
+    ) -> RebalanceResponse | RebalanceSimulationResponse:
+        """Calculate rebalancing for a portfolio."""
+        targets = {t.ticker: Decimal(str(t.target_ratio)) for t in portfolio.targets}
+        holdings = {h.ticker: (h.quantity, Decimal(str(h.avg_price))) for h in portfolio.holdings}
+
+        all_tickers = list(set(targets.keys()) | set(holdings.keys()))
+        current_prices = self.get_current_prices(all_tickers)
+        stock_names = self.get_stock_names(all_tickers)
+
+        # Calculate current values
+        current_values = {}
+        for ticker, (quantity, _) in holdings.items():
+            price = current_prices.get(ticker, Decimal("0"))
+            current_values[ticker] = price * quantity
+
+        current_total = sum(current_values.values())
+
+        if additional_amount:
+            new_total = current_total + additional_amount
+        else:
+            new_total = current_total
+
+        # Calculate rebalance items
+        items = []
+        for ticker in all_tickers:
+            target_ratio = targets.get(ticker, Decimal("0"))
+            current_value = current_values.get(ticker, Decimal("0"))
+            current_quantity = holdings.get(ticker, (0, Decimal("0")))[0]
+            current_price = current_prices.get(ticker, Decimal("0"))
+
+            if new_total > 0:
+                current_ratio = (current_value / new_total * 100).quantize(Decimal("0.01"))
+            else:
+                current_ratio = Decimal("0")
+
+            target_value = (new_total * target_ratio / 100).quantize(Decimal("0.01"))
+            diff_value = target_value - current_value
+
+            if current_price > 0:
+                diff_quantity = int(diff_value / current_price)
+            else:
+                diff_quantity = 0
+
+            if diff_quantity > 0:
+                action = "buy"
+            elif diff_quantity < 0:
+                action = "sell"
+            else:
+                action = "hold"
+
+            items.append(RebalanceItem(
+                ticker=ticker,
+                name=stock_names.get(ticker),
+                target_ratio=target_ratio,
+                current_ratio=current_ratio,
+                current_quantity=current_quantity,
+                current_value=current_value,
+                target_value=target_value,
+                diff_value=diff_value,
+                diff_quantity=diff_quantity,
+                action=action,
+            ))
+
+        # Sort by action priority (buy first, then sell, then hold)
+        action_order = {"buy": 0, "sell": 1, "hold": 2}
+        items.sort(key=lambda x: (action_order.get(x.action, 3), -abs(x.diff_quantity)))
+
+        if additional_amount:
+            return RebalanceSimulationResponse(
+                portfolio_id=portfolio.id,
+                current_total=current_total,
+                additional_amount=additional_amount,
+                new_total=new_total,
+                items=items,
+            )
+        else:
+            return RebalanceResponse(
+                portfolio_id=portfolio.id,
+                total_value=current_total,
+                items=items,
+            )