diff --git a/internal/handlers/expenses.go b/internal/handlers/expenses.go
index f4b7834..416f9d5 100644
--- a/internal/handlers/expenses.go
+++ b/internal/handlers/expenses.go
@@ -286,13 +286,22 @@ func (h *ExpenseHandler) SaveExpense(w http.ResponseWriter, r *http.Request) {
 	if convertedAmountStr != "" {
 		convertedAmount, _ = strconv.ParseFloat(convertedAmountStr, 64)
 	}
-	if baseCurrency == "" || baseCurrency == currency {
-		baseCurrency = currency
-		convertedAmount = amount
+
+	// Fetch the event to get its exchange rate for auto-calculation.
+	event, err := database.GetEventByID(h.DB, eventID)
+	if err != nil || event == nil || event.UserID != getUserID(r) {
+		http.Error(w, "Forbidden", http.StatusForbidden)
+		return
 	}
-	if convertedAmount <= 0 {
+
+	// Auto-calculate conversion.
+	if baseCurrency == "" || baseCurrency == currency {
+		baseCurrency = event.BaseCurrency
+	}
+	if convertedAmount <= 0 && baseCurrency != currency {
+		convertedAmount = amount * event.ExchangeRate
+	} else if convertedAmount <= 0 {
 		convertedAmount = amount
-		baseCurrency = currency
 	}
 
 	// 5. Build and save the expense record.
@@ -435,10 +444,7 @@ func (h *ExpenseHandler) UpdateExpense(w http.ResponseWriter, r *http.Request) {
 	amount, _ := strconv.ParseFloat(r.FormValue("amount"), 64)
 	convertedAmount, _ := strconv.ParseFloat(r.FormValue("converted_amount"), 64)
 	baseCurrency := r.FormValue("base_currency")
-	if baseCurrency == "" {
-		baseCurrency = r.FormValue("currency")
-		convertedAmount = amount
-	}
+	currency := r.FormValue("currency")
 
 	// Fetch the existing expense to preserve the event_id and image_path.
 	existing, err := database.GetExpenseByID(h.DB, expenseID)
@@ -460,6 +466,16 @@ func (h *ExpenseHandler) UpdateExpense(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	// Auto-calculate conversion.
+	if baseCurrency == "" {
+		baseCurrency = event.BaseCurrency
+	}
+	if convertedAmount <= 0 && baseCurrency != currency {
+		convertedAmount = amount * event.ExchangeRate
+	} else if convertedAmount <= 0 {
+		convertedAmount = amount
+	}
+
 	expense := database.Expense{
 		ID:              expenseID,
 		EventID:         existing.EventID,