What Is Purgeable Space on Mac, and How Do You Get It Back?

Your Mac says 40 GB are available. An installer says there is not enough room. Both are telling the truth, because a chunk of that 40 GB is purgeable: space macOS is holding for something useful and will hand back when it decides you need it.

Quick answer

Where the number comes from

Finder and Disk Utility report available space as “what you can use”, which includes storage the system is prepared to give up. The filesystem reports what is on disk right now. The gap between the two is purgeable.

df -h /System/Volumes/Data

Run that and compare it with the figure in Finder. On a Mac with a week of local snapshots, the difference lands in the tens of gigabytes.

What sits inside purgeable

Three sources cover almost every case.

• Local Time Machine snapshots. macOS keeps point-in-time copies on the internal disk, not only on your backup drive. They expire on their own and get thinned when space runs short, and they are usually the largest single contributor.
• Files kept offline by Optimize Mac Storage. Documents already uploaded to iCloud, watched TV purchases, old mail attachments. Their local copies can go because the originals live elsewhere.
• Caches the system treats as disposable. The same material described in Is It Safe to Delete ~/Library/Caches on Mac?, from the system’s point of view rather than yours.

When it becomes a real problem

macOS releases purgeable space when an app asks the filesystem for room and there is none. That works for everyday copying and downloading.

It fails in three situations:

• an installer or a game launcher checks free space upfront and refuses to start;
• Disk Utility cannot resize or erase a volume because the container has no headroom;
• a virtual machine or disk image needs a large contiguous allocation right away.

In those cases you have to release the space yourself.

Releasing local snapshots

Start by looking at what is there.

tmutil listlocalsnapshots /

The output lists snapshot names with their timestamps. Then ask macOS to reclaim a specific amount:

# release roughly 20 GB, at the highest urgency level
sudo tmutil thinlocalsnapshots / 21474836480 4

The number is bytes, and the trailing 4 is the urgency. Level 4 tells macOS to be aggressive rather than polite about which snapshots go.

To remove one specific snapshot instead:

sudo tmutil deletelocalsnapshots 2026-08-30-101500

Then check the free space again. On most Macs this single step closes the gap.

The other sources

Trash. Deleted files stay counted until the Trash is empty. Obvious, and still the answer often enough to check first.

Optimize Mac Storage. In System Settings, under General and then Storage, macOS offers to keep only recent files locally. Turning it off makes iCloud pull everything back down, which needs the space rather than freeing it, so use this one deliberately: the goal is to stop new offloading, not to reclaim anything today.

A restart. Some caches and temporary allocations only clear on boot. Cheap to try before anything more involved.

What not to do

Skip the tools that advertise a button for deleting purgeable space. There is nothing to delete that macOS will not release on request, and a cleaner that goes after snapshots without telling you is removing your fastest path back to a file you deleted this morning.

Writing a giant dummy file to force macOS into releasing space is the other trick that circulates. It works, and it also spends an hour of disk writes to accomplish what one tmutil command does in seconds.

Bottom line

Purgeable space means macOS is holding storage it can release on demand, mostly local snapshots. Leave it alone during normal use, because those snapshots are worth having. When a specific job needs room and cannot wait, list the snapshots, thin them with tmutil, empty the Trash, and measure again. If the gap survives all of that, the problem is ordinary files, and finding them is a different job.