diff --git a/README.md b/README.md
index e22c32e..22c0afe 100644
--- a/README.md
+++ b/README.md
@@ -14,6 +14,10 @@ Also, the future should include tests in this repo.
   Initial testing reduced build time from 5-10 minutes per image to
   15-30 seconds.
 
+  The `./size-test/build.sh` test is able to build an incremental
+  netboot image in just 12 seconds, and the initrd is not rebuilt.
+
+
 ## Setting up recursive nix
 
 Setting up recursive Nix requires support on the build machine:
diff --git a/size-test/.gitignore b/size-test/.gitignore
new file mode 100644
index 0000000..91ce5dd
--- /dev/null
+++ b/size-test/.gitignore
@@ -0,0 +1,3 @@
+netboot-base
+netboot-incremental
+netboot-incremental.drv
diff --git a/size-test/base.nix b/size-test/base.nix
new file mode 100644
index 0000000..f626af7
--- /dev/null
+++ b/size-test/base.nix
@@ -0,0 +1,3 @@
+{
+  imports = [ ../quickly.nix ];
+}
diff --git a/size-test/build.sh b/size-test/build.sh
new file mode 100755
index 0000000..ca5426c
--- /dev/null
+++ b/size-test/build.sh
@@ -0,0 +1,27 @@
+#!/bin/sh
+
+set -eux
+
+cd "$(dirname "$0")"
+
+nix-build '<nixpkgs/nixos>' \
+          -I nixos-config=./base.nix \
+          -A config.system.build.ipxeBootDir \
+          --out-link ./netboot-base
+
+nix-instantiate '<nixpkgs/nixos>' \
+          -I nixos-config=./incremental.nix \
+          -A config.system.build.ipxeBootDir \
+          --add-root ./netboot-incremental.drv --indirect
+
+time nix-build ./netboot-incremental.drv \
+     --out-link ./netboot-incremental
+
+ensureSame() (
+    test "$(realpath "./netboot-base/$1")" =  "$(realpath "./netboot-incremental/$1")"
+)
+
+ensureSame bzImage
+ensureSame initrd
+
+echo "ok!"
diff --git a/size-test/incremental.nix b/size-test/incremental.nix
new file mode 100644
index 0000000..e58ac57
--- /dev/null
+++ b/size-test/incremental.nix
@@ -0,0 +1,4 @@
+{
+  imports = [ ./base.nix ];
+  services.nginx.enable = true;
+}