diff --git a/inc/susa.h b/inc/susa.h
index 94ffc59..acf9d60 100644
--- a/inc/susa.h
+++ b/inc/susa.h
@@ -42,8 +42,10 @@
 #ifndef SUSA_H
 #define SUSA_H
 
+namespace susa {
 // Constants
-#define PI  3.1415926535897932384626433
+const double PI = 3.1415926535897932384626433;
+}
 
 // STL headers
 #include <cmath>