diff --git a/init/resize-rootfs-if-needed.sh b/init/resize-rootfs-if-needed.sh
index c3d21cb..2a93f1e 100755
--- a/init/resize-rootfs-if-needed.sh
+++ b/init/resize-rootfs-if-needed.sh
@@ -15,9 +15,7 @@ sysfs_xvda="/sys/class/block/xvda"
 
 # if root filesystem use already (almost) the whole dis
 non_rootfs_data=$(( 250 * 1024 * 2 ))
-rootfs_size=$(df --output=size / | tail -n 1)
-# convert to 512-byte blocks
-rootfs_size=$(( rootfs_size * 2 ))
+rootfs_size=$(df --block-size=512 --output=size / | tail -n 1)
 if [ "$(cat "$sysfs_xvda/size")" -lt \
        $(( non_rootfs_data + rootfs_size )) ]; then
    echo "root filesystem already at $rootfs_size blocks" >&2