diff --git a/include/xo/unit/xquantity.hpp b/include/xo/unit/xquantity.hpp
index 84807aa4..92e31a89 100644
--- a/include/xo/unit/xquantity.hpp
+++ b/include/xo/unit/xquantity.hpp
@@ -311,6 +311,26 @@ namespace xo {
             return Quantity::add(x, y);
         }
 
+        /** note: won't have constexpr result until c++26 (when ::sqrt(), ::pow() are constexpr)
+         **/
+        template <typename Quantity>
+        requires quantity2_concept<Quantity>
+        constexpr auto
+        operator+ (const Quantity & x, double y)
+        {
+            return x + Quantity(y, nu::dimensionless);
+        }
+
+        /** note: won't have constexpr result until c++26 (when ::sqrt(), ::pow() are constexpr)
+         **/
+        template <typename Quantity>
+        requires quantity2_concept<Quantity>
+        constexpr auto
+        operator+ (double x, const Quantity & y)
+        {
+            return Quantity(x, nu::dimensionless) + y;
+        }
+
         /** note: won't have constexpr result until c++26 (when ::sqrt(), ::pow() are constexpr)
          **/
         template <typename Quantity, typename Quantity2>