diff --git a/init/resize-rootfs-if-needed.sh b/init/resize-rootfs-if-needed.sh
index c94513c..77207cf 100755
--- a/init/resize-rootfs-if-needed.sh
+++ b/init/resize-rootfs-if-needed.sh
@@ -14,7 +14,9 @@ fi
 sysfs_xvda="/sys/class/block/xvda"
 
 # if root filesystem use already (almost) the whole dis
-size_margin=$(( 250 * 1024 * 2 ))
+# 203M for BIOS and /boot data, 222 for ext4 filesystem overhead
+# See QubesOS/qubes-core-agent-linux#146 for more details
+size_margin=$(((222 + 203) * 2 * 1024))
 rootfs_size=$(df --block-size=512 --output=size / | tail -n 1 | tr -d ' ')
 if [ $(cat $sysfs_xvda/size) -lt \
        $(( size_margin + rootfs_size )) ]; then