diff --git a/src/PolyTrader.Modules.CopyTrading/Logic/TradeMath.cs b/src/PolyTrader.Modules.CopyTrading/Logic/TradeMath.cs
new file mode 100644
index 0000000..cb1f228
--- /dev/null
+++ b/src/PolyTrader.Modules.CopyTrading/Logic/TradeMath.cs
@@ -0,0 +1,29 @@
+namespace PolyTrader.Modules.CopyTrading.Logic
+{
+    /// <summary>
+    /// Reine, geldkritische Trade-Mathematik (realisierter PnL, PnL-%, gewichteter
+    /// Einstiegspreis). Aus der Engine herausgezogen, damit diese Rechnungen fest
+    /// unit-getestet sind – Rundungs-/Vorzeichenfehler hier verfälschen direkt PnL und
+    /// Kontostand. Verhalten 1:1 aus der Engine übernommen (mit defensivem Null-Guard).
+    /// </summary>
+    public static class TradeMath
+    {
+        /// <summary>Erlös eines Verkaufs: Anzahl Shares × Verkaufspreis.</summary>
+        public static decimal ExitValue(decimal sizeShares, decimal exitPrice) => sizeShares * exitPrice;
+
+        /// <summary>Realisierter Gewinn/Verlust = Erlös − eingesetztes Kapital.</summary>
+        public static decimal RealizedPnl(decimal sizeShares, decimal exitPrice, decimal entryAmountUsd) =>
+            ExitValue(sizeShares, exitPrice) - entryAmountUsd;
+
+        /// <summary>PnL in Prozent des eingesetzten Kapitals; 0, wenn kein Kapital eingesetzt war.</summary>
+        public static decimal PnlPercent(decimal realizedPnl, decimal entryAmountUsd) =>
+            entryAmountUsd > 0m ? realizedPnl / entryAmountUsd * 100m : 0m;
+
+        /// <summary>
+        /// Gewichteter Durchschnitts-Einstiegspreis nach Aufstockung einer Position:
+        /// Gesamt-USDC / Gesamt-Shares. Bei Size ≤ 0 wird 0 zurückgegeben (Div-durch-0-Schutz).
+        /// </summary>
+        public static decimal WeightedAverageEntryPrice(decimal totalAmountUsd, decimal totalSize) =>
+            totalSize > 0m ? totalAmountUsd / totalSize : 0m;
+    }
+}
diff --git a/src/PolyTrader.Modules.CopyTrading/Services/CopyTradingEngine.cs b/src/PolyTrader.Modules.CopyTrading/Services/CopyTradingEngine.cs
index c272ea0..34f8020 100644
--- a/src/PolyTrader.Modules.CopyTrading/Services/CopyTradingEngine.cs
+++ b/src/PolyTrader.Modules.CopyTrading/Services/CopyTradingEngine.cs
@@ -497,7 +497,7 @@ namespace PolyTraderSharp.Services
                         {
                             old.Size += pos.Size;
                             old.AmountUsd += pos.AmountUsd;
-                            old.EntryPrice = old.AmountUsd / old.Size; // weighted average
+                            old.EntryPrice = TradeMath.WeightedAverageEntryPrice(old.AmountUsd, old.Size); // weighted average
                             return old;
                         });
 
@@ -541,7 +541,7 @@ namespace PolyTraderSharp.Services
                             {
                                 old.Size += pos.Size;
                                 old.AmountUsd += pos.AmountUsd;
-                                old.EntryPrice = old.AmountUsd / old.Size;
+                                old.EntryPrice = TradeMath.WeightedAverageEntryPrice(old.AmountUsd, old.Size);
                                 return old;
                             });
 
@@ -612,8 +612,8 @@ namespace PolyTraderSharp.Services
                         {
                             _positionRepo.DeleteDemo(account.AccountId, signal.TokenId);
 
-                            decimal exitUsd = openPos.Size * signal.Price;
-                            decimal realizedPnl = exitUsd - openPos.AmountUsd;
+                            decimal exitUsd = TradeMath.ExitValue(openPos.Size, signal.Price);
+                            decimal realizedPnl = TradeMath.RealizedPnl(openPos.Size, signal.Price, openPos.AmountUsd);
 
                             _state.GlobalPnl += realizedPnl;
                             account.UpdateBalance(account.AvailableBalance + exitUsd);
@@ -633,7 +633,7 @@ namespace PolyTraderSharp.Services
                                 ExitPrice = signal.Price,
                                 Size = openPos.Size,
                                 RealizedPnl = realizedPnl,
-                                PnlPercent = openPos.AmountUsd > 0 ? (realizedPnl / openPos.AmountUsd * 100m) : 0m,
+                                PnlPercent = TradeMath.PnlPercent(realizedPnl, openPos.AmountUsd),
                                 OpenedAt = openPos.OpenedAt,
                                 ClosedAt = DateTime.UtcNow,
                                 ExitReason = signal.Reason
diff --git a/tests/PolyTrader.Tests/TradeMathTests.cs b/tests/PolyTrader.Tests/TradeMathTests.cs
new file mode 100644
index 0000000..b335dda
--- /dev/null
+++ b/tests/PolyTrader.Tests/TradeMathTests.cs
@@ -0,0 +1,51 @@
+using PolyTrader.Modules.CopyTrading.Logic;
+using Xunit;
+using static PolyTrader.Modules.CopyTrading.Logic.TradeMath;
+
+namespace PolyTrader.Tests
+{
+    /// <summary>
+    /// Sicherheitsnetz für die geldkritische Trade-Mathematik (PnL, PnL-%, gewichteter Entry).
+    /// </summary>
+    public class TradeMathTests
+    {
+        [Fact]
+        public void ExitValue_is_shares_times_price()
+        {
+            Assert.Equal(99m, ExitValue(100m, 0.99m));
+        }
+
+        [Fact]
+        public void RealizedPnl_profit_and_loss()
+        {
+            Assert.Equal(49m, RealizedPnl(sizeShares: 100m, exitPrice: 0.99m, entryAmountUsd: 50m));
+            Assert.Equal(-20m, RealizedPnl(sizeShares: 100m, exitPrice: 0.40m, entryAmountUsd: 60m));
+        }
+
+        [Fact]
+        public void PnlPercent_relative_to_entry_capital()
+        {
+            Assert.Equal(98m, PnlPercent(realizedPnl: 49m, entryAmountUsd: 50m));
+            Assert.Equal(-20m, PnlPercent(realizedPnl: -20m, entryAmountUsd: 100m));
+        }
+
+        [Fact]
+        public void PnlPercent_zero_entry_returns_zero_not_divide_by_zero()
+        {
+            Assert.Equal(0m, PnlPercent(49m, 0m));
+        }
+
+        [Fact]
+        public void WeightedAverageEntryPrice_is_amount_over_size()
+        {
+            Assert.Equal(0.5m, WeightedAverageEntryPrice(totalAmountUsd: 100m, totalSize: 200m));
+            Assert.Equal(0.51m, WeightedAverageEntryPrice(51m, 100m));
+        }
+
+        [Fact]
+        public void WeightedAverageEntryPrice_zero_size_returns_zero()
+        {
+            Assert.Equal(0m, WeightedAverageEntryPrice(100m, 0m));
+        }
+    }
+}