Is it possible to figure out the size of a derivation after building it? For example, after nixos-rebuild, how much space does the new derivation take, excluding any storage savings because of previous builds?

  • One approach: Get a list of dependencies for each, calculate the ones in common and the ones specific to each. And then get the size of each dependency. Sep 1, 2021 at 15:51

2 Answers 2


Try this:

nix-store -q --requisites `nix-build --no-out-link '<nixpkgs>' -A vim` | sort -uf | xargs du -ch | tail -1

I also like to look at:

for p in /nix/var/nix/profiles/system* /nix/var/nix/profiles/per-user/$USER/profile*
   echo -n $p" ⇒ "
   nix-store -q --requisites $p | sort -uf | xargs du -ch | tail -1

Obviously the size of the transitive closure of derivations heavily overlap (shared libs, etc.) so the numbers will always add to more than the total space usage. Still useful though.

  • This is closer than the other answer, but it doesn't show where the derivations corresponding to the generations listed by nix-env --list-generations are stored.
    – l0b0
    Oct 10, 2021 at 20:45
  • They're pinned by the symlinks /nix/var/nix/profiles/per-user/$USER/profile*
    – obadz
    Oct 12, 2021 at 17:04
  • Oh, nice! That does it, then :) nix-store -q --requisites /nix/var/nix/profiles/per-user/$USER/profile-1-link | sort -uf | xargs du -ch | tail -1
    – l0b0
    Oct 12, 2021 at 21:32
  • Or even for path in /nix/var/nix/profiles/per-user/$USER/profile-*; do nix-store -q --requisites "$path" | xargs du --si --total | tail --lines=1 | sed "s#total#${path}#"; done
    – l0b0
    Oct 12, 2021 at 21:39

You may want nix path-info (e.g., for emacs):

$ nix-build -A emacs '<nixpkgs>'
$ nix path-info -Sh ./result
/nix/store/3l9lddwxz1mayaxvw8iy50ygzzfh1s1b-emacs-27.2   470.4M

From the Nix release notes:

nix path-info shows information about store paths, replacing nix-store -q. A useful feature is the option --closure-size (-S). For example, the following command show the closure sizes of every path in the current NixOS system closure, sorted by size:

nix path-info -rS /run/current-system | sort -nk2

See also the Nix manual on Installables.

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