diff --git a/include/xo/unit/Quantity.hpp b/include/xo/unit/Quantity.hpp
index c46552ea..dc846fda 100644
--- a/include/xo/unit/Quantity.hpp
+++ b/include/xo/unit/Quantity.hpp
@@ -195,6 +195,10 @@ namespace xo {
                 return x.scale() <=> y2.scale();
             }
 
+            Quantity operator-() const {
+                return Quantity(-scale_, unit_);
+            }
+
             /* also works with Quantity2 = double, int, .. */
             template <typename Quantity2>
             Quantity & operator*= (const Quantity2 & x) {
diff --git a/utest/Quantity.test.cpp b/utest/Quantity.test.cpp
index 15d52196..01ee661c 100644
--- a/utest/Quantity.test.cpp
+++ b/utest/Quantity.test.cpp
@@ -340,6 +340,10 @@ namespace xo {
                     auto q1_minus_q2 = q1 - q2;
                     REQUIRE(q1_minus_q2.unit() == q1.unit());
                     REQUIRE(q1_minus_q2.scale() == Approx(1.0 - k2).epsilon(1.0e-6));
+
+                    auto q1_neg = -q1;
+                    REQUIRE(q1_neg.unit() == q1.unit());
+                    REQUIRE(q1_neg.scale() == -q1.scale());
                 }
             }
         }