Purgeable Space: The Storage Your Mac Says You Have

Purgeable space is storage that's occupied right now but that macOS believes it can reclaim on demand. Apple counts it as available, because from the operating system's point of view it effectively is. Most third-party tools count it as used, because from the file system's point of view it is.

What macOS considers purgeable

• Time Machine local snapshots. Hourly APFS snapshots kept on your internal drive. macOS deletes the oldest automatically when space is needed.
• iCloud-optimised files. With “Optimise Mac Storage” enabled, files already uploaded to iCloud can be evicted locally and re-downloaded later.
• Caches macOS considers disposable, including some app caches registered with the system.
• Large downloads and temporary files that macOS has flagged as recreatable.

Why two tools give you two different answers

Run df -h in Terminal and compare it to Finder. They will frequently disagree by tens of gigabytes. Finder is showing you available-including-purgeable. df is showing you what's unallocated.

Neither number is wrong. They answer different questions: “how much can I probably use?” versus “how much is free?”

How to see the real number

The real figure comes from Terminal:

df -h /

The Avail column is free space. To see how much of the gap is snapshots:

tmutil listlocalsnapshots /

Each snapshot listed is holding storage that Finder is counting as yours.

Forcing macOS to release it

You can't delete purgeable space directly, and you should be sceptical of any tool claiming a magic button for it. What works:

1. Restart. Clears swap and prompts snapshot thinning.
2. Thin snapshots explicitly with tmutil thinlocalsnapshots / 10000000000 4, which asks macOS to free roughly 10 GB at urgency level 4.
3. Turn off “Optimise Mac Storage” temporarily if iCloud eviction is confusing your numbers.
4. Create genuine pressure. macOS reclaims aggressively when it truly needs to. Counter-intuitively, the system is usually right to wait.

The short conclusion

Purgeable space is a good feature that communicates badly. On a healthy Mac with reasonable headroom, ignore it, macOS is managing storage exactly as designed. Chase it only when you're hitting real disk-full errors, and then attack the underlying cause: genuine files you no longer need.

If your storage problem persists after a restart, the cause is almost never purgeable space. It is usually old iOS backups, duplicate files, or an app cache that has grown to an unreasonable size. All of which are visible if you look at the disk directly.