Glossary
Purgeable space
Disk space macOS counts as available because it can reclaim it on its own when needed.
What it is
Purgeable space holds things macOS can remove without asking: Time Machine local snapshots, caches, and iCloud files that are also stored online. It shows as available in Finder and Storage settings.
When an app needs room, macOS frees purgeable space first. You don't have to delete it by hand to install an update.
Why it matters
It explains why Finder, Storage settings and df can disagree about free space, and why a nearly full disk sometimes still installs a large update.
In Activity Monitor
Not shown. System Settings → General → Storage shows the space it can free.
In iKnowMyMac
The Storage map shows purgeable space as its own section, next to large files, old downloads and dev caches.
tmutil listlocalsnapshots /Lists Time Machine local snapshots, a common part of purgeable space.
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