Where did my Mac storage go?

First, why the numbers never agree

Before you can find the space, you have to stop being confused by the measurements. Ask three tools how much room is left and you will get three answers. This is not a bug. Each tool is answering a subtly different question.

The one that catches everyone is purgeable space. The Finder counts space that is technically occupied, by caches, local snapshots and evictable iCloud files, as "available," because macOS could free it if you needed it. That is a reasonable thing to show, but it means the Finder's free-space figure is an optimistic estimate, not a measurement. Howard Oakley, who has tested this carefully, once caught the Finder claiming 227 GB available when the real figure was 144 GB, an 83 GB error that matched a bogus purgeable estimate. His advice, and ours, is simple: for the honest number, use df or Disk Utility.

# the honest free-space number
$ df -h /

The method: map, don't guess

Once you trust a number, finding what fills the disk is a solved problem, and the solution is not to open folders one at a time wondering which is biggest. It is to see the whole disk sorted by size, so the largest things are impossible to miss. Two views do this well.

A size-sorted list shows folders ranked largest first, so you drill straight into the heaviest branch, see what is big inside it, and repeat. A treemap draws every folder as a rectangle sized by how much space it uses, turning your whole disk into one picture where the big consumers are the big boxes. Both beat guessing, because they direct your attention to where the space actually is.

Work from the top down. Start at the volume root, go into the biggest box, then the biggest box inside that, and keep going until you reach something you recognise and can act on, an old video export, a forgotten disk image, a developer cache, an iPhone backup. The usual suspects on a full Mac are large media files, developer and creative app caches, Docker, iPhone backups, and local snapshots.

The Mac-specific trap: double-counting

Here is something most generic disk tools get wrong on a Mac, and it can make their numbers nonsense. Since macOS Catalina, your boot disk is split into two volumes: a read-only System volume and a writable Data volume, stitched together so they look like one. The stitching uses a special link type Apple calls a firmlink, which, unlike ordinary links, works in both directions across the volume boundary.

The consequence for measurement is sharp: a tool that naively walks every directory can cross a firmlink and traverse the same files a second time, counting them twice. The result is a size tool that cheerfully reports more data on your disk than the disk can physically hold. Correctly measuring a Mac means being aware of the volume layout and not following firmlinks into double-counting.

The second trap: clones

APFS has a feature called cloning: two files can share the same underlying data blocks until one of them changes, so a "copy" can cost almost nothing. Modern developer tools lean on this heavily, pnpm, uv, git worktrees and others create clones deliberately to save space. The problem is that the classic du command does not understand clones. It reports each clone's full logical size as though it were a separate copy, so a folder full of clones can look ten or a hundred times bigger than the space it truly occupies.

So if du tells you a directory is enormous but deleting things barely moves df, clones may be why: the space was shared all along. A tool that detects clone families and counts shared blocks once gives you the real figure.

A reliable procedure

1. Get the honest free-space number with df -h / or Disk Utility. Ignore the Finder's figure.
2. If a recent deletion "did nothing," suspect local snapshots and check tmutil listlocalsnapshots /.
3. Open a size-sorted view or treemap of the whole volume.
4. Drill from the top into the largest branches until you reach things you recognise.
5. Remove only what you understand and can get back: caches, build output, old exports, stale backups. Leave originals and system files alone.

That is the entire game. Most "my Mac is mysteriously full" situations dissolve the moment you stop trusting the wrong number and start looking at a map instead of guessing.

Sources

• Apple — WWDC 2019 "What's New in Apple File Systems" (System/Data volume split and firmlinks); on-device manual pages man du, man df, man getattrlist.
• Howard Oakley, The Eclectic Light Company — on where macOS gets its free-space figures, purgeable space, and inaccurate Finder reporting; APFS clones.
• Open-source work documenting clone-aware disk measurement on APFS.