diff --git a/export-image/prerun.sh b/export-image/prerun.sh
index 3acc248..72cb299 100755
--- a/export-image/prerun.sh
+++ b/export-image/prerun.sh
@@ -14,7 +14,7 @@ TOTAL_SIZE=$(du --apparent-size -s ${EXPORT_ROOTFS_DIR} --exclude var/cache/apt/
 
 ROUND_SIZE="$((4 * 1024 * 1024))"
 ROUNDED_ROOT_SECTOR=$(((2 * BOOT_SIZE + ROUND_SIZE) / ROUND_SIZE * ROUND_SIZE / 512 + 8192))
-IMG_SIZE=$(((BOOT_SIZE + TOTAL_SIZE + (800 * 1024 * 1024) + ROUND_SIZE) / ROUND_SIZE * ROUND_SIZE))
+IMG_SIZE=$(((BOOT_SIZE + TOTAL_SIZE + (800 * 1024 * 1024) + ROUND_SIZE - 1) / ROUND_SIZE * ROUND_SIZE))
 
 truncate -s ${IMG_SIZE} ${IMG_FILE}
 fdisk -H 255 -S 63 ${IMG_FILE} <<EOF