What purgeable space on a Mac is, and how to get it back
Why Finder, Storage settings and Terminal disagree about free space, what macOS counts as purgeable, and how to make local snapshots give space back now.
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Purgeable space is space macOS counts as available because it can clear it by itself the moment something needs room. Local Time Machine snapshots are the big one, followed by iCloud files that are already safe in iCloud and caches that apps have told macOS they can live without. It's real space. It just isn't empty yet.
That's why the free-space number depends on who you ask. Finder's Get Info window shows something like "Available: 80 GB (35 GB purgeable)", Storage settings draws its own bar, and df in Terminal usually shows the smallest number of the three.
Four readings of the same free space
Here's my MacBook Air at 22:05 on September 28, 2026, measured four ways within the same minute:
| Where | What it reported |
|---|---|
df -H / (decimal units, like Finder) | 3.45 GB available |
df -h / (binary units) | 3.2 GiB available: the same space, counted in 1,073,741,824-byte gigabytes |
diskutil info / | Container Free Space: 3.4 GB |
| Apple's API for "space available for important use" | 3.55 GB |
The first three are the same raw free space in different units and roundings. The last one adds what macOS is prepared to purge, so the difference, about 100 MB here, is my purgeable space. It's tiny because my disk is nearly full: macOS had already cleared most of what it could, and Apple's own support note says Time Machine only keeps local snapshots on disks with plenty of free space. On a Mac with room to spare, the gap is often tens of gigabytes.
What counts as purgeable
- Local Time Machine snapshots. With Time Machine on, macOS saves a snapshot of the startup disk about every hour and keeps each one for 24 hours, plus one for the last successful backup and one before each macOS update. Apple is explicit that your Mac counts the space used by snapshots as available storage.
- Optimized iCloud files. With Optimize Mac Storage on for iCloud Drive or Photos, macOS can remove local copies of things that are safely in iCloud and fetch them again when you open them.
- Caches that apps mark as purgeable through the system's cache-management service. Plenty of apps never do this, which is why a big
~/Library/Cachesfolder isn't necessarily purgeable at all.
Do you need to do anything?
Usually not. When something asks for space (a large copy, a download, an update), macOS purges first and the operation carries on. Apple's note on snapshots says as much: they don't use the space needed for downloading, copying or installing.
It goes wrong in two situations. Some installers and virtual machine tools check raw free space before they start and refuse, without giving macOS the chance to purge anything. And on a nearly full disk, like mine, there's simply nothing left to purge: the 100 MB above won't save anyone. At that point you need to delete something real, and the order I'd do that in is a separate post.
How to get it back now
Start by seeing whether you have any local snapshots at all:
tmutil listlocalsnapshots /If the list is long and you need the space now, ask macOS to thin them. The number is how many bytes you want back (20 GB here) and the last argument is urgency, from 1 to 4:
sudo tmutil thinlocalsnapshots / 20000000000 4This only touches the snapshots on your Mac. Backups on your Time Machine drive are left alone. To delete one particular snapshot, use its date from tmutil listlocalsnapshotdates:
sudo tmutil deletelocalsnapshots 2026-09-28-101500If you'd rather not use Terminal, Apple's own method works too: in System Settings → General → Time Machine, click Options, set Backup Frequency to Manually, wait a few minutes while the local snapshots are deleted, then set it back. Time Machine remembers your backup disks.
For iCloud Drive, Control-click a folder you don't need offline and choose Remove Download. The files stay in iCloud and the local copies go.
Why no app can tell you exactly how big a snapshot is
APFS snapshots share storage with your live files. A snapshot only costs the blocks that have changed or been deleted since it was taken, and macOS doesn't give you that figure per snapshot in any tool I'd trust. Deleting a snapshot might return 40 GB or 40 MB, and you find out afterwards.
So if a cleaner shows "Snapshots: 23.4 GB" as a precise number, it's estimating. Spaceback doesn't show numbers it can't measure, and for snapshots the honest tool is tmutil.
Vlada Misici builds Spaceback, a storage cleaner for Mac and iPhone. Every number in these posts was measured on a real Mac, and every command was run before it went on the page. Found a mistake? Email vlada@queazy.app and it gets fixed.