diff --git a/units.h b/units.h
index 0e168a21e..0a9e44b79 100644
--- a/units.h
+++ b/units.h
@@ -1,6 +1,8 @@
 #ifndef UNITS_H
 #define UNITS_H
 
+#include <math.h>
+
 #ifdef __cplusplus
 extern "C" {
 #endif