What is purgeable space on a Mac, and why does it make your disk look full?

Select your Mac's disk in Finder, press ⌘I, and the Get Info window says something like Available: 48 GB (31 GB purgeable). Run df -h in Terminal and it says 17 GB. An installer then tells you there is not enough room. All three are telling the truth. They count different things.

What purgeable means

Purgeable space is storage that is in use right now but that macOS is allowed to free whenever something needs the room. It is not a folder. It is a label macOS puts on certain files. On most Macs, almost all of it comes from three places:

• Time Machine local snapshots. If you back up with Time Machine, macOS keeps snapshots on the internal disk between backups. They are deleted when space runs short, or after about 24 hours.
• iCloud Drive files kept on your Mac. With Optimize Mac Storage on, files you have opened that are already safe in iCloud can have their local copy removed, so they count as purgeable. Photos does the same with full-size originals when its own Optimize option is on.
• Caches macOS manages. Some system and app caches are marked as safe to remove under pressure.

The idea is sound. Unused space does nothing for you, so macOS fills it with things that save time later (a backup you can restore from, a file that opens without downloading) and gives it back when you need it.

Where to see it

Where	What it shows
Finder › select the disk › ⌘I	Available, with the purgeable part in brackets
Finder status bar (View › Show Status Bar)	Available space, which includes purgeable
System Settings › General › Storage	Available space, and the categories using the rest
df -h / in Terminal	Only what is free right now, without purgeable
tmutil listlocalsnapshots / in Terminal	The local snapshots behind part of the purgeable figure

Different tools draw the line in different places, which is why no two of them quite agree. If two numbers on your Mac disagree, purgeable space is usually the gap between them.

Why the disk can still be "full"

macOS frees purgeable space when it notices that something needs it. Most of the time that works. Some apps, however, check the free space before they start, see only what df sees, and refuse. Large installers, macOS updates, virtual machines and video software are the usual ones. So you get "not enough space" with a Finder window showing 40 GB available.

There is a second case. If you are running out of space quickly, for example while copying a large file, macOS may not purge fast enough to keep up.

Turning purgeable space into free space

Do these only when something needs real free space now. Otherwise, leave purgeable space alone and let macOS manage it.

1. Thin the local snapshots. In Terminal:

tmutil listlocalsnapshots /
tmutil thinlocalsnapshots / 21474836480 4

The second command asks macOS to free up to 20 GB (the number is in bytes) of snapshots at the highest urgency. It removes the oldest first, and it affects only the copies on your Mac, not the backups on your Time Machine disk. Run your next backup soon afterwards.

2. Remove local copies of iCloud Drive files. In Finder, right-click a file or folder in iCloud Drive and choose Remove Download. The file stays in iCloud and downloads again the next time you open it. Do this for large folders you have finished with.

3. Restart. Some temporary files and snapshots are cleared on restart. It is crude, but it works often enough to try first.

What not to do: do not fill the disk on purpose with a huge file to force a purge, a trick that circulates online. If something goes wrong halfway, you are left with a full disk.

Real space, not purgeable space

If your available space is low even counting purgeable, the problem is not purgeable space. It is the files themselves. Start with your Mac's storage is almost full, and if a grey bar is the biggest thing on the chart, what is System Data.

Doing it with SweepyMac

SweepyMac does not delete Time Machine snapshots, and we think that is right for a cleaner. Snapshots are a safety net, macOS already manages them, and the tmutil command above does the job when you really need it. Here is what SweepyMac does instead.

• Cloud Cleanup does step 2 for every iCloud Drive file at once. It removes the local copies of files you have already downloaded, but only files that are fully uploaded, and it reports what was actually freed. The files stay in iCloud and download again when you open them. It does not go through the Trash, so there is no Undo, only opening the file again. Getting disk space back from iCloud Drive covers it in detail.
• Disk is a read-only overview of what is using the disk: Applications, Photos, Movies, Downloads, App Data and so on. What it cannot measure goes into System & Other rather than being guessed at.
• Space Lens finds what is using space that is not purgeable, as a map you can drill into.
• Smart Scan clears caches, logs and saved app state, which is real space rather than space macOS will free one day. Everything goes to the Trash with Undo.

SweepyMac has a three-day free trial with no account, then from $14.99 a year or $59.96 once. Download it from its page.