Purgeable space on macOS explained: dux

What purgeable means

Purgeable space is disk space that is in use right now, but that macOS is allowed to clear on its own the moment something more important needs it. Think of it as a reserve: the data sits on your disk because there is room for it, and it disappears when there is not.

Because macOS can reclaim it at any time, it counts purgeable space as available. That is why Finder and Storage settings often report more free space than the disk technically has. Apple says this directly about Time Machine snapshots: your Mac counts the space they use as available storage.

What is inside purgeable space

Purgeable is not a folder you can open. It is a label macOS puts on data it knows it can recreate or fetch again. On most Macs it comes from a handful of places:

Time Machine local snapshots. When Time Machine backs up automatically, your Mac keeps hourly snapshots on the startup disk. They are usually the largest part of purgeable space. Local snapshots, explained covers them in detail.
iCloud Drive files with Optimize Mac Storage. Downloaded files that are safely in iCloud can be turned back into online only placeholders when space runs low. See iCloud Drive, Optimize Mac Storage and what’s really on disk.
Media and attachments you already have elsewhere. With the Optimize Storage recommendation on, watched Apple TV movies and shows, and older Mail attachments, can be removed and downloaded again later.
Caches marked as purgeable. macOS and some apps flag cached data, such as downloaded updates or app content, so the system can clear it under pressure.

Where you see it

There are three places, and they rarely agree.

Storage settings. Open System Settings, choose General, then Storage. The number at the top is what macOS considers available, which includes purgeable space.
Finder. Select your startup disk (usually Macintosh HD) and choose File, then Get Info. Available space here also includes purgeable space. Turn on View, then Show Status Bar to see the same figure at the bottom of every Finder window.
Terminal. The df command reports only space that is free right now. It does not count purgeable space at all.

~ % df -h /System/Volumes/Data
Filesystem    Size  Used  Avail Capacity iused ifree %iused  Mounted on
/dev/disk3s1  926Gi 701Gi 188Gi   79%    3.1M  2.0G    0%   /System/Volumes/Data

The gap between the Avail column and what Finder reports is, roughly, your purgeable space. On a Mac with Time Machine and a busy iCloud Drive, that gap can be tens or even hundreds of gigabytes. On your Mac, the numbers will differ.

Why it does not free up on command

Purgeable space is freed on demand, not on a schedule. macOS clears it when an app asks for space and the free space alone is not enough. It does this in small steps, oldest and least useful data first, and only as much as the request needs.

That design is efficient, but it has side effects you may notice:

Some installers check the wrong number. An app or installer that looks only at truly free space (like df does) can refuse to start, even though Storage settings shows plenty of room.
Large copies can stall. Copying a big file onto a nearly full disk can pause while macOS clears purgeable data in the background.
The number keeps moving. New snapshots appear every hour and old ones expire, so purgeable space grows and shrinks during the day.

There is no button in macOS that says “purge now”. The safe options below get you close.

How to free purgeable space safely

Start with the gentle steps. Each one is reversible or harmless.

1. Restart your Mac

A restart clears swap files and many temporary files, and gives macOS a moment to tidy up caches. It does not delete snapshots, but it often frees a few gigabytes on its own.

2. Thin Time Machine local snapshots

If you use Time Machine, snapshots are usually most of the purgeable space. First, check whether you have any:

~ % tmutil listlocalsnapshots /
Snapshots for disk /:
com.apple.TimeMachine.2026-10-29-215507.local
com.apple.TimeMachine.2026-10-29-225520.local
# ask macOS to reclaim up to 50 GB, at the highest urgency
~ % tmutil thinlocalsnapshots / 50000000000 4

The second command tells Time Machine to thin snapshots until it has reclaimed the amount you give (in bytes), with an urgency from 1 to 4. Your backups on the external disk are not touched. If you prefer not to use Terminal, Apple’s own method works too: open System Settings, choose General, then Time Machine, click Options and set the backup frequency to Manually. Wait a few minutes, then switch it back.

3. Remove downloads of iCloud files you do not need locally

In Finder, Control-click a large folder in iCloud Drive and choose Remove Download. The files stay in iCloud and download again when you open them. This turns purgeable space into free space straight away.

4. Use the Storage recommendations

In Storage settings, the recommendations at the top include Optimize Storage (watched Apple TV content and older Mail attachments) and Empty Trash Automatically. They help macOS keep more space in the reclaimable pool.

5. Start up in safe mode once

Apple notes that starting your Mac in safe mode clears certain system caches, which can free enough space to finish a task such as a macOS update. Restart normally afterwards.

When purgeable space is worth worrying about

Most of the time, it is not. Purgeable space is doing its job when it shrinks as you need room. It is worth acting on when:

an installer or app reports too little space, but Storage settings says you have plenty;
free space in df stays below about 10 percent of your disk for days, even after a restart and a completed Time Machine backup;
you deleted large files and the space never came back (a snapshot from before the deletion is holding on to those files).

If none of these apply, leave purgeable space alone. It is space you already have, waiting to be used.