Purgeable space on a Mac: what it is, why the free space numbers disagree, and why not to fight it

What purgeable space actually is

APFS, the file system every Mac has used since 2017, lets macOS mark some data as disposable: keep it while there is room, drop it the moment the room is needed. That data is "purgeable". It is a category, not a folder, and it is made up of three main things.

• Time Machine local snapshots. When Time Machine is on, macOS takes a snapshot of the startup disk every hour and keeps the last 24, plus one a day, on the disk itself, so you can restore a file you deleted an hour ago even when the backup drive is not connected. A snapshot costs nothing when it is taken; it grows as you change and delete files, because it keeps the old versions. Delete a 50 GB folder and the space does not come back until the snapshot that remembers it is thinned. Snapshots are usually the largest part of purgeable space, and the only part with a visible list: tmutil listlocalsnapshots / in Terminal. The Time Machine local snapshots page covers them in full.
• iCloud files kept locally. With Optimize Mac Storage on for iCloud Drive or Photos, macOS keeps full copies of files that are also in iCloud, because it has room. Those copies are purgeable: evict them and they download again when opened.
• Caches and downloadable assets. Fonts, dictionaries, Siri voices, GarageBand sounds, Safari's cached media, app caches flagged by their developers as safe to purge. Smaller, but it adds up.

The idea is sound. An empty disk is wasted; a disk full of things you might want, which vanish silently when you need the space, is better. It only confuses people because macOS has never agreed with itself about whether that space is free.

Why Finder, Disk Utility and the Storage pane disagree

Here is the same disk, measured three ways. These are typical figures for a 500 GB Mac with Time Machine on, not from one specific machine; Howard Oakley's test linked in the sources found gaps of over 90 GB between views on his.

Apple's own documentation says, in a support note quoted in the Apple Community thread below, that Finder and Get Info do not include local snapshots in their calculations of the storage space available on a disk, while the Storage pane and Disk Utility handle them differently again. The practical reading: the Storage pane and Finder tell you what you could have, Disk Utility tells you what you have right now, and a large file copy will sometimes believe Disk Utility. That is the whole mystery of "my Mac says there is no space when there is free space".

Why there is "no free space" when Finder shows plenty

The failure looks like this. Finder says 90 GB available. You try to copy a 20 GB file, or install a macOS update, or export a video, and macOS says the disk is full. Disk Utility says 15 GB free and 75 GB purgeable. Nobody is lying; the purge did not happen in time.

macOS purges reactively. When a write needs more room than is really free, the system starts thinning snapshots and evicting iCloud copies, but that takes time, sometimes minutes for a big snapshot, and the write that triggered it may give up first. Some operations check free space up front using the strict number and never trigger a purge at all. Installers are the classic case: a macOS update that needs 25 GB will refuse with 15 GB really free and 75 GB purgeable, and succeed an hour later when the snapshots have thinned.

There is a second, nastier version. A snapshot can be far larger than you expect if it was taken just before a huge deletion, because it is holding every byte you deleted. The Apple Community thread quoted below had a 416 GB snapshot behind a 492 GB System Data figure; Oakley's reader in the 2026 article had a 350 GB one. Neither showed up anywhere in the Storage pane except as a lump of System Data that would not shrink. tmutil listlocalsnapshots / shows that the snapshot exists; the dates in its names tell you which one it is.

How purgeable space is reclaimed, and what speeds it up

macOS thins purgeable space on four triggers, and all four are things you can bring about safely.

1. Low free space. When real free space drops below a threshold (it varies with disk size, but think a few gigabytes to a few percent), macOS starts thinning the oldest local snapshots and evicting the least recently used iCloud copies. This is the designed path and it happens without you.
2. Time. Hourly snapshots expire after 24 hours; daily ones are kept while there is room and thinned when there is not. Delete a big folder today, and the snapshot holding it is gone by this time tomorrow. Most "the space came back on its own" stories are this.
3. A completed Time Machine backup. Local snapshots exist to cover the gap between backups. Connect the backup drive, let a backup finish, and the snapshots it has now copied are released. This is the fastest honest way to clear a large snapshot.
4. A big write. Copying a large file to the disk forces a purge if the copy is willing to wait for it, which Finder usually is. Oakley's "Friday Magic" article, linked in the sources, turned 228 GB of purgeable into used space exactly this way, by copying files until macOS gave the space up.

What does not help: restarting (snapshots survive it), deleting more files (the snapshot keeps them), emptying the Trash again, or clearing caches, which are the smallest slice. The free up space guide covers the deletions that reduce the really-used number, which is the number that matters.

What you should not do: delete snapshots by hand with tmutil or Disk Utility as a routine step. It works, and plenty of guides recommend it, but the snapshot is also your only local backup of the last 24 hours, and the deletion is the kind of command where one wrong character removes the wrong thing. If a single giant snapshot is blocking an update and you cannot wait a day, a completed Time Machine backup releases it cleanly, and that is the route we recommend.

What to do when purgeable space is in the way, in order

Safest first. Most people are done at step three.

1. Check it really is purgeable

   Open Disk Utility, select Macintosh HD (the Data volume if it is listed separately), and read the Available figure and the purgeable figure in brackets. If purgeable is small and the disk is simply full, this page is not your problem; the free up space guide is.

2. See what the snapshots are

   Open Terminal and type tmutil listlocalsnapshots / then press Return. It only lists; it changes nothing and needs no password. Each line is a snapshot named with its date and time. One dated just before a large deletion is the one holding the space.

3. Free some real space

   Empty the Trash, delete old iPhone backups, remove unused apps. Reducing really-used space lowers pressure and lets macOS begin thinning snapshots on its own. The Storage pane updates within a few minutes.

4. Wait 24 hours

   Hourly snapshots expire after a day. Leave the Mac on and plugged in overnight. Check Disk Utility in the morning; the purgeable number is usually a fraction of what it was.

5. Run a Time Machine backup

   If you use Time Machine, connect the drive and let a backup complete (Time Machine menu, Back Up Now). Local snapshots that are now on the drive are released straight away. This is the quickest safe way to clear a very large snapshot.

6. Copy the big file anyway

   If a copy or install fails for space while purgeable is large, try it again. The first attempt often triggers the purge and the second succeeds. For a macOS installer, download it, wait an hour, and run it again.

7. If a giant snapshot remains

   After a backup and a day's wait, a snapshot still holding hundreds of gigabytes is unusual. Turn Time Machine off and on again in System Settings, General, Time Machine, which prompts macOS to re-evaluate its snapshots, then run a backup. If it persists, Apple Support can walk through removing it; that is a better place to type a deletion command than a blog comment.

Why not to fight purgeable space

Once people learn that purgeable space can be forced out, some try to keep it at zero: turning off local snapshots, scripting deletions, running cleaners that "optimize" by evicting iCloud files. All of it trades something useful for a number on a screen.

Local snapshots are the reason you can recover the file you deleted at 2pm when you realize at 4pm, even on a MacBook that has not seen its backup drive in a week. They are the reason a failed macOS update can roll back. Evicting iCloud copies means every file you open downloads first, on a connection that may not be there. Caches that were purged get rebuilt, which is CPU time and heat. And the space you forced free is not yours to keep: the moment macOS has room, it starts keeping useful things in it again, because that is what the design is for.

The healthy picture of a Mac's disk is: really-used space comfortably under about 85 percent, a purgeable slice of whatever size macOS wants, and a really-free margin of at least 10 to 15 GB so swap, updates and new snapshots never stall. Get the first number right by deleting the real things listed in the free up space guide, and let the other two look after themselves. If the Storage pane's System Data figure is the thing that looks wrong, the System Data page explains how snapshots end up counted there.

Safe to Quit shows real free space and the purgeable slice side by side in the menu bar, so the number you see is the one the next file copy will see, which removes most of the surprise.