Purgeable space on a Mac, and why you cannot delete it

Two numbers, same disk, wildly different answers. It is the single most confusing thing about storage on a modern Mac, and once you know what purgeable space is, most of the confusion goes away.

The short answer

Purgeable space is disk space holding files that macOS has decided it can delete without asking you, the instant something else needs the room. Local Time Machine snapshots. Cached copies of files that also live in iCloud. Caches the system knows how to rebuild.

df counts it as used, because right now those files exist. Finder counts it as free, because the moment you try to copy a 40 GB video onto the disk, macOS quietly deletes enough of it to make room and the copy succeeds. Both are telling the truth about different questions.

Seeing how much you have

Compare the two numbers directly.

df -h /System/Volumes/Data

df reports the raw filesystem number, which counts purgeable files as used because right now they exist. Now open System Settings → General → Storage, or check a Finder window’s status bar, and you will usually see a larger figure — macOS adds the purgeable space back on, because it knows it can free it on demand.

The gap between the two is your purgeable space. On the Mac used for this article, df reports 179.6 GB available while macOS tells Finder 185.6 GB, so roughly 6 GB is purgeable.

Finder spells it out, if you know where to look. Open a Finder window on your startup disk, then View → Show Status Bar. On many versions the status bar reads “X GB available” with the purgeable portion named directly.

Why Apple built it this way

APFS, the filesystem on every modern Mac, makes snapshots cheap. A snapshot records the state of the disk at a moment in time, and it costs almost nothing at first, growing only as files change afterwards.

Time Machine leans on this heavily. Even with no backup drive attached, macOS takes hourly snapshots and keeps them for 24 hours, so you can recover a file you deleted this morning without any external disk. iCloud does something similar with files marked “Optimize Mac Storage”: the file is in the cloud, and the local copy is a convenience that can be dropped.

Both of these are genuinely useful. Both of them look like your disk filling up.

When it actually matters

Most of the time, purgeable space is a non-problem, and the correct response is to do nothing. Three situations are the exception.

An installer refuses to start. Some installers, including macOS updates, check the raw filesystem number rather than asking macOS what it could free, and refuse before anything gets a chance to be purged. Frustrating, and the fix is to force the purge yourself.

Disk utilities and disk images. Creating a large disk image or partitioning sometimes needs contiguous real space and will not trigger a purge.

You are trying to diagnose something. If you are trying to work out where 200 GB went, purgeable space muddies every number you look at until you clear it.

Forcing macOS to hand it back

The honest way is to thin the snapshots, which is where most purgeable space lives:

tmutil listlocalsnapshots /
sudo tmutil thinlocalsnapshots / 50000000000 4

The number is bytes to reclaim — about 50 GB here. Urgency 4 is the most aggressive.

To remove one specific snapshot:

tmutil deletelocalsnapshots 2026-09-01-120000

For iCloud-backed purgeable space, turn off Optimize Mac Storage in System Settings → [your name] → iCloud → iCloud Drive temporarily, or just open the files you want kept locally.

You will also find advice to create a huge dummy file to force a purge, usually with mkfile or dd. It works, because the disk genuinely fills and macOS purges to cope. It is also a blunt instrument that risks filling the disk for real if you get the size wrong. Thin the snapshots instead.

What not to worry about

You cannot delete purgeable space directly, and there is no folder to look in. That is not a bug — the whole point is that macOS owns the decision. Any app claiming it can “clean purgeable space” is either thinning snapshots (which you can do in one line) or lying.

If your disk is genuinely full and thinning snapshots did not help, then the space is real and something else is using it. The System Data article covers where to look next.