diff --git a/include/xo/unit/Quantity.hpp b/include/xo/unit/Quantity.hpp
index c94dbebe..299e0d2d 100644
--- a/include/xo/unit/Quantity.hpp
+++ b/include/xo/unit/Quantity.hpp
@@ -110,6 +110,31 @@ namespace xo {
                 }
             }
 
+            template <typename Quantity2>
+            static constexpr
+            auto subtract(const Quantity & x, const Quantity2 & y) {
+                using r_repr_type = std::common_type_t<typename Quantity::repr_type,
+                                                       typename Quantity2::repr_type>;
+                using r_int_type = std::common_type_t<typename Quantity::ratio_int_type,
+                                                      typename Quantity2::ratio_int_type>;
+
+                /* conversion to get y in same units as x:  multiply by y/x */
+                auto rr = detail::nu_ratio(y.unit(), x.unit());
+
+                if (rr.natural_unit_.is_dimensionless()) {
+                    r_repr_type r_scale = (static_cast<r_repr_type>(x.scale())
+                                           - (::sqrt(rr.outer_scale_sq_)
+                                              * rr.outer_scale_exact_.template to<r_repr_type>()
+                                              * static_cast<r_repr_type>(y.scale())));
+
+                    return Quantity<r_repr_type, r_int_type>(r_scale, x.unit_.template to_repr<r_int_type>());
+                } else {
+                    /* units don't match! */
+                    return Quantity<r_repr_type, r_int_type>(std::numeric_limits<Repr>::quiet_NaN(),
+                                                             x.unit_.template to_repr<r_int_type>());
+                }
+            }
+
         private:
             /** @brief quantity represents this multiple of a unit amount **/
             Repr scale_ = Repr{};
@@ -167,6 +192,16 @@ namespace xo {
             return Quantity::add(x, y);
         }
 
+        /** note: won't have constexpr result until c++26 (when ::sqrt(), ::pow() are constexpr)
+         **/
+        template <typename Quantity, typename Quantity2>
+        requires quantity2_concept<Quantity> && quantity2_concept<Quantity2>
+        constexpr auto
+        operator- (const Quantity & x, const Quantity2 & y)
+        {
+            return Quantity::subtract(x, y);
+        }
+
         namespace unit {
             constexpr auto nanogram = natural_unit_qty(nu2::nanogram);
         }
diff --git a/utest/unit.test.cpp b/utest/unit.test.cpp
index 80be39db..09ddcea1 100644
--- a/utest/unit.test.cpp
+++ b/utest/unit.test.cpp
@@ -1158,6 +1158,45 @@ namespace xo {
 
             //REQUIRE(ng2.scale() == 1);
         } /*TEST_CASE(Quantity6)*/
+
+        TEST_CASE("Quantity7", "[Quantity]") {
+            constexpr bool c_debug_flag = true;
+
+            // can get bits from /dev/random by uncommenting the 2nd line below
+            //uint64_t seed = xxx;
+            //rng::Seed<xoshio256ss> seed;
+
+            //auto rng = xo::rng::xoshiro256ss(seed);
+
+            scope log(XO_DEBUG2(c_debug_flag, "TEST_CASE.Quantity7"));
+            //log && log("(A)", xtag("foo", foo));
+
+            /* not constexpr until c++26 */
+            Quantity ng = unit_qty(su2::nanogram);
+            Quantity ug = unit_qty(su2::microgram);
+
+            {
+                auto sum1 = ng - ug;
+                log && log(xtag("ng-ug", sum1));
+
+                /* units will be nanograms,  since that's on lhs */
+                REQUIRE(sum1.unit().n_bpu() == 1);
+                REQUIRE(sum1.unit()[0].scalefactor() == scalefactor_ratio_type(1, 1000000000));
+                REQUIRE(sum1.scale() == -999.0);
+            }
+
+            {
+                auto sum2 = ug - ng;
+                log && log(xtag("ug-ng", sum2));
+
+                /* units will be micrograms,  since that's on rhs */
+                REQUIRE(sum2.unit().n_bpu() == 1);
+                REQUIRE(sum2.unit()[0].scalefactor() == scalefactor_ratio_type(1, 1000000));
+                REQUIRE(sum2.scale() == 0.999);
+            }
+
+            //REQUIRE(ng2.scale() == 1);
+        } /*TEST_CASE(Quantity7)*/
     } /*namespace ut*/
 } /*namespace xo*/