diff --git a/src/curvature.c b/src/curvature.c
index 70f6af2..a5c0187 100644
--- a/src/curvature.c
+++ b/src/curvature.c
@@ -17,6 +17,9 @@
  * Boston, MA 02111-1307, USA.
  */
 
+#ifdef _WIN32
+#define _USE_MATH_DEFINES
+#endif
 #include <math.h>
 #include "gts.h"
 
diff --git a/src/triangle.c b/src/triangle.c
index 1d82869..47170c6 100644
--- a/src/triangle.c
+++ b/src/triangle.c
@@ -17,6 +17,9 @@
  * Boston, MA 02111-1307, USA.
  */
 
+#ifdef _WIN32
+#define _USE_MATH_DEFINES
+#endif
 #include <math.h>
 #include "gts.h"