Glossary/Figures Activity Monitor leaves out
Purgeable space
Also: purgeable storage
Quick answer
Purgeable space is disk space macOS counts as available but has not freed yet: files it can delete on its own when something needs the room. Local Time Machine snapshots, caches, and copies of files that also live in iCloud, Apple Music or the TV app are the usual contents.
It is why two tools disagree about free space. Finder's Get Info on the disk shows Available with purgeable space included, and the storage settings say the same. A command like `df`, an installer's check, or an app that asks the file system directly sees only what is free right now, which can be tens of gigabytes less.
In theory that never matters, because macOS purges as soon as a write needs the space. In practice a big copy or an installer can fail with a full-disk error while Finder says 40 GB is available, because the purge did not happen in time or the space it would free was not enough.
How to see how much is purgeable
In Finder, select Macintosh HD and press ⌘I. The Available line reads something like 120 GB (30 GB purgeable). System Settings → General → Storage shows the same figure and, under System Data, part of what makes it up.
Local snapshots are often the biggest piece on a laptop with Time Machine on. Terminal lists them with tmutil listlocalsnapshots /. macOS thins them when the disk runs short, and deletes them once the backup disk has a copy.
When a full-disk error appears while space shows as available
Wait a minute and try again; macOS purges in the background and the second attempt often succeeds. If it does not, remove the local snapshots with tmutil deletelocalsnapshots followed by the snapshot's date from the list, or turn Time Machine off and on, which clears them.
Emptying the Trash, restarting, and letting Optimize Mac Storage move old files to iCloud all free real space rather than purgeable space. Third-party cleaners mostly delete caches that macOS would have purged anyway, so they change the figure but not the outcome.
Questions people ask
Is purgeable space free space?
Not until macOS frees it. It is space macOS could free by deleting snapshots, caches and cloud copies, and it usually does so when a write needs it. Tools that ask the file system directly do not count it.
How do I clear purgeable space on a Mac?
Delete local Time Machine snapshots with `tmutil deletelocalsnapshots`, empty the Trash, and restart. Most caches are cleared by macOS itself the moment space is needed, so there is little to gain from deleting them by hand.
Why does my Mac say the disk is full when Finder shows space available?
The available figure includes purgeable space that has not been freed yet. The app or installer that failed asked for real free space and did not get it. Wait and retry, or delete the local snapshots that usually make up the difference.
Related
Want to watch this all the time? See the best system monitors for Mac, or the best Activity Monitor alternatives. Or browse every term in the glossary.