Time Machine local snapshots: what they are, why they fill System Data, and when to delete them

What a local snapshot actually is

Since macOS High Sierra, the startup disk uses APFS, and APFS can take a snapshot: a read-only, frozen view of the whole volume at one instant. A snapshot is not a copy of your files. It is a bookmark. The disk keeps the blocks that were in use at that moment, and only the blocks that change afterwards take up extra space. Taking a snapshot is instant and costs nothing at first; it grows only as you change or delete files that the snapshot still refers to.

Time Machine uses this to give you a backup even when the backup drive is in a drawer. With Time Machine switched on, macOS takes a local snapshot of the startup disk roughly once an hour, and keeps each one for up to 24 hours. Apple's description is plain: Time Machine makes copies, each hour, of files that have changed, and those copies are stored for up to 24 hours. It also takes one extra snapshot right before installing a macOS update, so the update can be rolled back.

The process doing this is backupd, the Time Machine engine. The same hourly cycle that writes to your external drive also takes the local snapshot, and when the external drive is away, only the local half happens.

Why it matters for you: if you delete a 20 GB folder, the space does not come back immediately. The most recent snapshot still references those blocks, so they stay on disk until that snapshot ages out, up to 24 hours later. That is the single most common reason "I deleted a lot and my free space did not change".

Why local snapshots show up as System Data or purgeable space

macOS has two ideas of free space. Finder and Get Info report space that is free or could be freed on demand, which includes snapshots. Apple's own note on local snapshots says that Finder and Get Info "don't include local snapshots in their calculations of the storage space available on a disk": in other words, they treat snapshot space as available. Disk Utility shows the same thing as purgeable.

The Storage panel in System Settings is different. It tries to assign every byte to a category, and snapshot blocks have no app or document to belong to, so they land in System Data. That is why System Data can read 80 GB one afternoon and 30 GB the next morning with no action from you: a day's worth of snapshots thinned overnight.

The growth can be real, though. One Apple Community thread describes finding that "a snapshot was occupying 416GB of disk space when it should not have been", on a Mac where the user had been moving large video files around. Every move or delete of a large file while a snapshot references it keeps the old blocks alive. Heavy file churn (video editing, large downloads, virtual machine images) makes snapshots balloon within their 24 hour window.

Howard Oakley, who has written the clearest explanations of APFS snapshots, warns that "Snapshots will only grow in size over time. Don't leave them lying around." The built-in 24 hour thinning is what stops that happening on a normal Mac. Problems start when something prevents thinning, or when snapshots other than Time Machine's exist (some backup and cloning apps make their own, and those are not subject to the 24 hour rule).

How macOS thins snapshots on its own

Two rules keep snapshots under control, and both run without you.

Age. Each Time Machine local snapshot is deleted when it is 24 hours old. On a quiet Mac that is the only rule you will ever see at work.

Space. If the disk gets low on free space, macOS deletes local snapshots early, oldest first, to make room. Apple describes snapshot space as space that is available to you, and that is what it means: when you need it, the system takes it back. Saving a large file, installing an app, or downloading a macOS update can all trigger it. This is why you will sometimes see free space jump up in the middle of an install.

What thinning does not do: it does not touch snapshots made by other software, and it cannot delete a snapshot that the system is currently using (for example, during a backup in progress). If a cloning app left snapshots behind, they stay until that app, or you, removes them.

A day of snapshots on a normal Mac

How to see your local snapshots

Open Terminal (Applications, Utilities) and run:

tmutil listlocalsnapshots /

You get a list like com.apple.TimeMachine.2026-11-03-091512.local, one line per snapshot, with the date and time in the name. A normal Mac with Time Machine on shows somewhere between a handful and about 24. Zero means Time Machine is off or has not run yet. A list with entries older than 24 hours, or entries whose names do not start with com.apple.TimeMachine, means something other than the 24 hour rule is in play.

This command only reads. It needs no password and changes nothing, so it is safe to run as often as you like. Disk Utility also shows snapshots: select the startup volume, then View, Show APFS Snapshots, and each one appears with its size. That size is the amount only that snapshot holds, so adding them up tells you what you would get back if they were all gone.

To see how much space snapshots and other purgeable items are holding in total, select the startup disk in Finder, press ⌘I, and compare Available with the number in brackets marked purgeable. Fuller breakdown on the purgeable space page.

When deleting local snapshots is reasonable, and the safe way

Most of the time, never. The 24 hour rule and the low-space rule reclaim the space on their own, and the snapshots are the only backup you have when the drive is unplugged. Deleting them by hand trades a day of file history for space you would have got back anyway.

Deleting is reasonable when:

• You need the space now, for example to finish an export or install an update, and the Storage panel shows tens of GB in System Data that Finder counts as available.
• You moved or deleted very large files and the snapshot from before the move is holding all of it.
• You are about to erase or hand over the Mac anyway.

The safe ways, in order of preference:

1. Wait. Up to 24 hours. Nothing to do.
2. Turn off Back Up Automatically for an hour. System Settings, General, Time Machine, Options. When automatic backups are off, macOS removes the existing local snapshots on its own within a short while. Turn it back on afterwards and Time Machine resumes, with a fresh snapshot at the next hour. This is reversible, needs no Terminal, and is the method Apple's own documentation points to.
3. Delete a specific snapshot with tmutil. tmutil deletelocalsnapshots followed by the date from the list removes one snapshot, and takes an administrator account. Use it only when you know which snapshot is holding the space (Disk Utility's snapshot view shows sizes) and only on Time Machine's own snapshots. Leave snapshots from other apps to those apps.

The trade-off is simple. With snapshots, a file you deleted or overwrote at 11:00 can be restored from the 10:00 snapshot through Time Machine's Browse Time Machine Backups, even with no drive attached. Without them, you have only what is on your external drive from its last connection. Most people never use local snapshots for recovery, and most people also never need to delete them. Let the system run them.

What to do, in order

1. List them

   In Terminal run tmutil listlocalsnapshots /. Count the entries and check the dates. Under 25, all within the last day: normal.

2. See what they hold

   Open Disk Utility, select the startup volume, then View, Show APFS Snapshots. The size column shows what each one would free.

3. If you can wait, wait

   Snapshots older than 24 hours delete themselves. Deleted files reappear as free space when the last snapshot referencing them goes.

4. If you need space now, pause automatic backups

   System Settings, General, Time Machine, Options, turn off Back Up Automatically. Give it a while, check Finder's free space, then turn it back on.

5. Only then use tmutil deletelocalsnapshots

   For one specific snapshot you have identified by date, as an administrator. Never remove snapshots that are not Time Machine's.

6. If System Data is still large

   Snapshots were not the main cause. Work through the System Data page, which covers caches, logs, old iOS backups and the rest.

Three things people get wrong about local snapshots

"They are the same as my Time Machine backup." No. The external backup keeps hourly, daily and weekly history for as long as the drive has room. Local snapshots keep 24 hours and live on the same disk as your files, so a dead SSD takes them with it. Oakley's note applies: "There's no relation between TM backup size and snapshot size."

"They are why my disk is full." Sometimes, briefly. But if the Mac is persistently low on space, snapshots are not the cause, because macOS thins them when space runs low. Persistent fullness is real data: large apps, downloads, caches, old backups. See what is in System Data.

"I should disable local snapshots." The old tmutil disablelocal command was removed in High Sierra, and there is no supported switch now except turning Time Machine off altogether. There is also no reason to want one: snapshots cost nothing until files change, and the system reclaims them under pressure. The right move is to leave them on and understand what the numbers mean.