diff --git a/src/include/units/physical/si/length.h b/src/include/units/physical/si/length.h
index 212c57b6..8dc939bf 100644
--- a/src/include/units/physical/si/length.h
+++ b/src/include/units/physical/si/length.h
@@ -34,6 +34,8 @@ struct centimetre : prefixed_unit<centimetre, centi, metre> {};
 struct decimetre : prefixed_unit<decimetre, deci, metre> {};
 struct kilometre : prefixed_unit<kilometre, kilo, metre> {};
 
+struct astronomical_unit : named_scaled_unit<astronomical_unit, "au", no_prefix, ratio<149'597'870'700>, metre> {};
+
 struct dim_length : physical::dim_length<metre> {};
 
 template<Unit U, Scalar Rep = double>
@@ -61,6 +63,10 @@ constexpr auto operator"" dm(long double l) { return length<decimetre, long doub
 constexpr auto operator"" km(unsigned long long l) { return length<kilometre, std::int64_t>(l); }
 constexpr auto operator"" km(long double l) { return length<kilometre, long double>(l); }
 
+// au
+constexpr auto operator"" au(unsigned long long l) { return length<astronomical_unit, std::int64_t>(l); }
+constexpr auto operator"" au(long double l) { return length<astronomical_unit, long double>(l); }
+
 }  // namespace literals
 
 }  // namespace units::si
diff --git a/test/unit_test/static/si_test.cpp b/test/unit_test/static/si_test.cpp
index 69205887..840a6778 100644
--- a/test/unit_test/static/si_test.cpp
+++ b/test/unit_test/static/si_test.cpp
@@ -55,6 +55,7 @@ static_assert(1km == 1000m);
 static_assert(1m == 100cm);
 static_assert(1m == 10dm);
 static_assert(1m == 1000mm);
+static_assert(1au == 149'597'870'700m);
 static_assert(1km + 1m == 1001m);
 static_assert(10km / 5km == 2);
 static_assert(100mm / 5cm == 2);