Time Machine local snapshots are eating your disk

This is the single most common cause of “System Data is 80 GB and I cannot find why”, and almost nobody knows the feature exists.

Time Machine does not only write to an external drive. Roughly every hour it also takes a snapshot of your startup disk and stores it on that same disk, so that you can recover a file you deleted this morning without plugging anything in. macOS keeps these for 24 hours, or until space runs short.

The feature is genuinely useful. It has saved me a document I deleted an hour before a deadline. It is also invisible, uncapped in any way you can configure, and reported as System Data.

Seeing what you have

tmutil listlocalsnapshots /

You get either a list of timestamps like com.apple.TimeMachine.2026-09-05-081500.local, or a bare header meaning you have none.

Working out how much space they occupy is harder than it should be, because APFS snapshots share storage with the live filesystem. A snapshot taken a minute ago costs almost nothing. One from yesterday costs whatever has changed since. There is no honest per-snapshot figure, which is why the usual approach is to thin them and see how much comes back.

Thinning them

The blunt instrument:

sudo tmutil thinlocalsnapshots / 50000000000 4

Two arguments after the mount point. The first is how many bytes you want back — 50000000000 is roughly 50 GB. The second is urgency, from 1 to 4, where 4 means “delete as much as needed to reach the target”.

To delete one specific snapshot:

tmutil deletelocalsnapshots 2026-09-01-120000

Note the date format: no com.apple.TimeMachine. prefix and no .local suffix, just the timestamp.

Check the result:

df -h /System/Volumes/Data

You are not deleting your backups

Worth saying plainly, because it stops people doing this.

Snapshots on the internal disk are a local convenience copy. The backups on your Time Machine drive are separate files on a separate disk and are not touched by any command above. Delete every local snapshot and your Time Machine history is exactly as it was.

The only thing you lose is the ability to restore a file from the last 24 hours without connecting the backup drive.

Should you turn it off entirely?

You can:

sudo tmutil disable

I would not. That switches off Time Machine altogether, which is a large price for a storage problem with an easier answer. Snapshots are purgeable — macOS deletes them automatically when space gets tight, so on a healthy machine this manages itself.

Thinning by hand is worth doing in two situations: an installer refusing to start because it checks free space before macOS gets a chance to purge, and when you are trying to work out where your disk went and want the numbers to stop moving.

If you want a real reduction in what Time Machine keeps, exclude things instead: System Settings → General → Time Machine → Options, then add the folders you do not need backed up. Virtual machine images, Docker data and downloads directories are the usual candidates, and excluding them shrinks both the snapshots and the backups.

The related thing that confuses people

If you have snapshots, Storage settings will report more free space than df does, sometimes by a lot. That gap is purgeable space, and snapshots are most of it. Neither number is wrong: df counts a snapshot as used because it occupies blocks, and macOS counts it as free because it can delete it the moment you need the room.