diff --git a/const.hh b/const.hh
index ab4acdc..342fd0b 100644
--- a/const.hh
+++ b/const.hh
@@ -19,6 +19,7 @@ struct Const {
 	static constexpr T FourPi()	{ return  12.56637061435917295385; } // 16*a(1)
 	static constexpr T SqrtPi()	{ return   1.77245385090551602730; } // sqrt(4*a(1))
 	static constexpr T SqrtTwoPi()	{ return   2.50662827463100050242; } // sqrt(8*a(1))
+	static constexpr T InvSqrtTwo()	{ return   0.70710678118654752440; } // sqrt(2)/2
 };
 
 }
diff --git a/fft.hh b/fft.hh
index 0110bdd..e3643a1 100644
--- a/fft.hh
+++ b/fft.hh
@@ -15,7 +15,7 @@ namespace FFT {
 template <typename TYPE>
 static inline TYPE rsqrt2(TYPE a)
 {
-	return (std::sqrt(typename TYPE::value_type(2)) / typename TYPE::value_type(2)) * a;
+	return Const<TYPE>::InvSqrtTwo() * a;
 }
 
 template <typename TYPE>