diff --git a/src/interfaces/product-entity.interface.go b/src/interfaces/product-entity.interface.go
index 6401687..931c0c9 100644
--- a/src/interfaces/product-entity.interface.go
+++ b/src/interfaces/product-entity.interface.go
@@ -56,3 +56,8 @@ func CreateNewProduct(productBase *ProductBaseStruct, calcSnapShot *CalcSnapshot
 	b.createNewId()
 	return b
 }
+
+func Calculate(symbol float64, snapshot *CshPremiumBuySellSnapshot) float64 {
+	price := symbol + float64(snapshot.Premium)
+	return price * (1 + float64(snapshot.Tax)/100)
+}
diff --git a/src/interfaces/product-entity.interface_test.go b/src/interfaces/product-entity.interface_test.go
new file mode 100644
index 0000000..661f2dc
--- /dev/null
+++ b/src/interfaces/product-entity.interface_test.go
@@ -0,0 +1,39 @@
+package interfaces
+
+import (
+	"testing"
+)
+
+func TestCalculate(t *testing.T) {
+	snapshot := &CshPremiumBuySellSnapshot{
+		Premium: 10,
+		Tax:     5,
+	}
+
+	t.Run("Positive values calculation", func(t *testing.T) {
+		symbol := 100.0
+		expected := 115.5
+		result := Calculate(symbol, snapshot)
+		if result != expected {
+			t.Errorf("Expected %f, but got %f", expected, result)
+		}
+	})
+
+	t.Run("Negative values calculation", func(t *testing.T) {
+		symbol := -50.0
+		expected := -42.0
+		result := Calculate(symbol, snapshot)
+		if result != expected {
+			t.Errorf("Expected %f, but got %f", expected, result)
+		}
+	})
+
+	t.Run("Zero values calculation", func(t *testing.T) {
+		symbol := 0.0
+		expected := 10.5
+		result := Calculate(symbol, snapshot)
+		if result != expected {
+			t.Errorf("Expected %f, but got %f", expected, result)
+		}
+	})
+}