diff --git a/fastmath.hpp b/fastmath.hpp
index bdf7ebf..f4f2342 100644
--- a/fastmath.hpp
+++ b/fastmath.hpp
@@ -44,6 +44,8 @@ struct decimal {
         return {MUL_F(d1.i, d2.i)};
     }
 
+    decimal &operator*=(const decimal &d) { return (*this) = {MUL_F(i, d.i)}; }
+
     friend decimal operator/(const decimal &d1, const decimal &d2) {
         return {DIV_F(d1.i, d2.i)};
     }