diff --git a/maths.hpp b/maths.hpp
index b0b511e6..40389da6 100644
--- a/maths.hpp
+++ b/maths.hpp
@@ -167,20 +167,29 @@ exactly_zero [[gnu::pure]] (T a)
 
 //-----------------------------------------------------------------------------
 // angles, trig
+
+template <typename T>
+struct constants { };
+
 constexpr double PI_d = 3.141592653589793238462643;
 constexpr float  PI_f = 3.141592653589793238462643f;
 
-constexpr float E_f = 2.71828182845904523536028747135266250f;
+constexpr float  E_f = 2.71828182845904523536028747135266250f;
+constexpr double E_d = 2.71828182845904523536028747135266250;
 
-constexpr double
-to_degrees [[gnu::pure]] (double radians) {
-    return radians * 180 / PI_d;
+template <typename T>
+constexpr T
+to_degrees [[gnu::pure]] (T radians)
+{
+    return radians * 180 / constants<T>::PI;
 }
 
 
-constexpr float
-to_degrees [[gnu::pure]] (float radians) {
-    return radians * 180 / PI_f;
+template <typename T>
+constexpr T
+to_radians [[gnu::pure]] (T degrees)
+{
+    return degrees / 180 * constants<T>::PI;
 }
 
 
diff --git a/maths.ipp b/maths.ipp
index 186d81a4..482aaacc 100644
--- a/maths.ipp
+++ b/maths.ipp
@@ -43,3 +43,20 @@ pow (T x, unsigned y)
 {
     return y == 0 ? 1 : x * pow (x, y - 1);
 }
+
+
+//-----------------------------------------------------------------------------
+template <>
+struct constants<float>
+{
+    static constexpr float PI = PI_f;
+    static constexpr float E  = E_f;
+};
+
+
+template <>
+struct constants<double>
+{
+    static constexpr double PI = PI_d;
+    static constexpr double E  = E_d;
+};