diff --git a/make-btrfs-fs.nix b/make-btrfs-fs.nix
index 10acc68..2762bc3 100644
--- a/make-btrfs-fs.nix
+++ b/make-btrfs-fs.nix
@@ -58,13 +58,9 @@ pkgs.stdenv.mkDerivation {
       # Make a crude approximation of the size of the target image.
       # If the script starts failing, increase the fudge factors here.
       numInodes=$(find ./rootImage | wc -l)
-      numDataBlocks=$(du -s -c -B 4096 --apparent-size ./rootImage | tail -1 | awk '{ print int($1 * 1.10) }')
-      bytes=$((2 * 4096 * $numInodes + 4096 * $numDataBlocks))
-      echo "Creating a btrfs image of $bytes bytes (numInodes=$numInodes, numDataBlocks=$numDataBlocks)"
 
-      truncate -s $bytes $img
-
-      faketime -f "1970-01-01 00:00:01" fakeroot mkfs.btrfs -L ${volumeLabel} -U ${uuid} -r ./rootImage $img
+      truncate -s 4G $img
+      faketime -f "1970-01-01 00:00:01" fakeroot mkfs.btrfs --shrink -L ${volumeLabel} -U ${uuid} -r ./rootImage $img
 
       if [ ${builtins.toString compressImage} ]; then
         echo "Compressing image"