diff --git a/math/emacs-calc/Makefile b/math/emacs-calc/Makefile
index 586de8804e13..249249c8a2a2 100644
--- a/math/emacs-calc/Makefile
+++ b/math/emacs-calc/Makefile
@@ -18,6 +18,10 @@ COMMENT=	Arbitrary-precision calculator for Emacs
 RUN_DEPENDS=	emacs:${PORTSDIR}/editors/emacs
 BUILD_DEPENDS=	emacs:${PORTSDIR}/editors/emacs
 
+USE_REINPLACE=	yes
 INFO=		calc
 
+post-patch:
+	@${REINPLACE_CMD} -e "s|%%PREFIX%%|${PREFIX}|g" ${WRKSRC}/Makefile
+
 .include <bsd.port.mk>
diff --git a/math/emacs-calc/files/patch-Makefile b/math/emacs-calc/files/patch-Makefile
index 1a20da7024c9..b53505e10b2c 100644
--- a/math/emacs-calc/files/patch-Makefile
+++ b/math/emacs-calc/files/patch-Makefile
@@ -22,7 +22,7 @@
  
 +# FreeBSD installation
 +
-+prefix        = /usr/local
++prefix        = %%PREFIX%%
 +infodir       = ${prefix}/info
 +sharedir      = ${prefix}/share
 +emacssharedir = ${sharedir}/emacs