diff --git a/autodiff.hh b/autodiff.hh
index de4e02d9..62b4758c 100644
--- a/autodiff.hh
+++ b/autodiff.hh
@@ -13,6 +13,8 @@
   that file for usage examples
  */
 
+// Apparently I need this in MSVC to get constants
+#define _USE_MATH_DEFINES
 #include <math.h>
 #include <string.h>
 #include "strides.h"
diff --git a/cameramodel-parser.re b/cameramodel-parser.re
index 3da87337..37b8d1cf 100644
--- a/cameramodel-parser.re
+++ b/cameramodel-parser.re
@@ -8,6 +8,10 @@
 //
 //     http://www.apache.org/licenses/LICENSE-2.0
 
+
+// Apparently I need this in MSVC to get constants
+#define _USE_MATH_DEFINES
+
 #include <stdio.h>
 #include <sys/types.h>
 #include <sys/stat.h>
diff --git a/mrcal.c b/mrcal.c
index 705920d6..fbd0b92b 100644
--- a/mrcal.c
+++ b/mrcal.c
@@ -6,6 +6,9 @@
 //
 //     http://www.apache.org/licenses/LICENSE-2.0
 
+// Apparently I need this in MSVC to get constants
+#define _USE_MATH_DEFINES
+
 #include <stdio.h>
 #include <stdlib.h>
 #include <inttypes.h>
diff --git a/poseutils-opencv.c b/poseutils-opencv.c
index dbecb1df..a0df80b5 100644
--- a/poseutils-opencv.c
+++ b/poseutils-opencv.c
@@ -30,6 +30,9 @@
 // and on any theory of liability, whether in contract, strict liability,
 // or tort (including negligence or otherwise) arising in any way out of
 
+// Apparently I need this in MSVC to get constants
+#define _USE_MATH_DEFINES
+
 #include <math.h>
 #include <float.h>
 
diff --git a/poseutils.c b/poseutils.c
index ad520866..b17563e9 100644
--- a/poseutils.c
+++ b/poseutils.c
@@ -6,6 +6,9 @@
 //
 //     http://www.apache.org/licenses/LICENSE-2.0
 
+// Apparently I need this in MSVC to get constants
+#define _USE_MATH_DEFINES
+
 #include <stdio.h>
 #include <string.h>
 #include <math.h>
diff --git a/stereo.c b/stereo.c
index 01d48a98..f9197519 100644
--- a/stereo.c
+++ b/stereo.c
@@ -6,6 +6,9 @@
 //
 //     http://www.apache.org/licenses/LICENSE-2.0
 
+// Apparently I need this in MSVC to get constants
+#define _USE_MATH_DEFINES
+
 #include <math.h>
 
 #include "mrcal.h"
diff --git a/test/test-cahvor.c b/test/test-cahvor.c
index 494918d9..3812a179 100644
--- a/test/test-cahvor.c
+++ b/test/test-cahvor.c
@@ -1,3 +1,6 @@
+// Apparently I need this in MSVC to get constants
+#define _USE_MATH_DEFINES
+
 #include <stdio.h>
 #include <stdlib.h>
 #include <assert.h>
diff --git a/test/test-harness.h b/test/test-harness.h
index 0b2a0040..2cb88792 100644
--- a/test/test-harness.h
+++ b/test/test-harness.h
@@ -1,5 +1,8 @@
 #pragma once
 
+// Apparently I need this in MSVC to get constants
+#define _USE_MATH_DEFINES
+
 #include <stdio.h>
 #include <stdbool.h>
 #include <math.h>