diff --git a/src/GTools.hpp b/src/GTools.hpp
index 69330e3..184b40c 100644
--- a/src/GTools.hpp
+++ b/src/GTools.hpp
@@ -25,6 +25,7 @@
 const double pi      = 3.1415926535897931159979635;
 const double pihalf  = pi/2.0;
 const double twopi   = 2.0*pi;
+const double fourpi  = 4.0*pi;
 const double deg2rad = 0.0174532925199432954743717;
 const double rad2deg = 57.295779513082322864647722;
 const double ln10    = 2.3025850929940459010936138;