diff --git a/target/linux/ar71xx/image/Makefile b/target/linux/ar71xx/image/Makefile
index 60990b284d..8ac70448d5 100644
--- a/target/linux/ar71xx/image/Makefile
+++ b/target/linux/ar71xx/image/Makefile
@@ -31,6 +31,10 @@ define mkcmdline
 $(if $(1),board=$(1) )$(if $(2),console=$(2)$(COMMA)$(3))
 endef
 
+define mtdpartsize
+$(shell echo $$((`echo '$(2)' | sed -e 's/.*[:$(COMMA)]\([0-9]*\)k[@]*[0-9a-zx]*($(1)).*/\1/'` * 1024)))
+endef
+
 SINGLE_PROFILES:=
 
 define SingleProfile