diff --git a/subsurface-core/units.h b/subsurface-core/units.h
index 1273bd9bb..9d1b0fc8c 100644
--- a/subsurface-core/units.h
+++ b/subsurface-core/units.h
@@ -2,6 +2,9 @@
 #define UNITS_H
 
 #include <math.h>
+#ifndef M_PI
+#define M_PI 3.14159265358979323846
+#endif
 
 #ifdef __cplusplus
 extern "C" {