diff --git a/core/Makefile b/core/Makefile
index 4d151c5..edde15f 100644
--- a/core/Makefile
+++ b/core/Makefile
@@ -707,13 +707,15 @@
 mkyaffs2_extra_flags := -c $(BOARD_NAND_PAGE_SIZE)
 else
 mkyaffs2_extra_flags :=
+BOARD_NAND_PAGE_SIZE := 2048
 endif
 
 ifneq ($(BOARD_NAND_SPARE_SIZE),)
 mkyaffs2_extra_flags += -s $(BOARD_NAND_SPARE_SIZE)
+else
+BOARD_NAND_SPARE_SIZE := 64
 endif
 
-
 # -----------------------------------------------------------------
 # system image
 #
diff --git a/core/definitions.mk b/core/definitions.mk
index c1a627b..9595d82 100644
--- a/core/definitions.mk
+++ b/core/definitions.mk
@@ -1777,10 +1777,11 @@
 
 # Convert a partition data size (eg, as reported in /proc/mtd) to the
 # size of the image used to flash that partition (which includes a
-# 64-byte spare area for each 2048-byte page).
+# spare area for each page).
 # $(1): the partition data size
 define image-size-from-data-size
-$(shell echo $$(($(1) / 2048 * (2048+64))))
+$(shell echo $$(($(1) / $(BOARD_NAND_PAGE_SIZE) * \
+  ($(BOARD_NAND_PAGE_SIZE)+$(BOARD_NAND_SPARE_SIZE)))))
 endef
 
 # $(1): The file(s) to check (often $@)