diff --git a/src/include/const.h b/src/include/const.h
index 88281c886..6c8ec45c4 100644
--- a/src/include/const.h
+++ b/src/include/const.h
@@ -12,6 +12,7 @@ Author: 1985 Thomas L. Quarles
 #define CONSTboltz (1.3806226e-23)
 #define CONSTepsZero (8.854214871e-12)   /* epsilon zero F/m */
 #define CONSTepsSiO2 (3.4531479969e-11)  /* epsilon SiO2 F/m */
+#define CONSTmuZero  (1.25663706143592e-6)         /* MuZero H/m       */
 #define REFTEMP 300.15 /* 27 degrees C */
 
     extern double CONSTroot2;
diff --git a/src/include/defines.h b/src/include/defines.h
index cee4f19c6..c85aa755a 100644
--- a/src/include/defines.h
+++ b/src/include/defines.h
@@ -64,6 +64,7 @@
 #define CONSTboltz (1.3806226e-23)
 #define CONSTepsZero (8.854214871e-12)   /* epsilon zero F/m */
 #define CONSTepsSiO2 (3.4531479969e-11)  /* epsilon SiO2 F/m */
+#define CONSTmuZero  (1.25663706143592e-6)         /* MuZero H/m       */
 
 #define REFTEMP 300.15 /* 27 degrees C */