What Is Purgeable Space on Mac?
Finder says the disk has 321 GB available. Storage settings shows a large grey band
labelled purgeable. df reports a different number again, in a different unit. None of
the three is lying, and none of them is answering quite the same question.
Almost every "my Mac says it is full but I cannot find the files" problem turns out to be a measurement problem before it is a storage problem. Knowing which question each number answers resolves most of them.
Short answer: purgeable space is space macOS believes it can reclaim if something needs it, mostly local snapshots, evictable cloud files, and caches. It is counted as available by Finder and not by every other tool. You cannot free it directly with a command, and you do not usually need to: macOS releases it on demand.
Three numbers, three questions
Run these on any Mac and you will get three different figures for the same volume.
df -h /
diskutil info / | grep -i "Container"
On the machine used for this article:
/dev/disk3s1s1 926Gi 12Gi 299Gi 4% /
Container Total Space: 994.6 GB (994610155520 Bytes)
Container Free Space: 321.0 GB (321045377024 Bytes)
df says 299Gi available. diskutil says 321.0 GB free. Those are the same quantity:
321 decimal gigabytes is 299 binary gibibytes. Apple reports storage in decimal GB,
while df -h prints binary units and labels them Gi. Before concluding that space has
vanished, check whether you are comparing GB against GiB.
The Used column reading 12Gi is the other surprise. On modern macOS, / is the sealed,
read-only system snapshot. Your files live on the data volume, which shares the same
APFS container. That is why per-volume "used" figures do not add up to the container.
| Question you are asking | The tool that answers it |
|---|---|
| How much can I write right now | diskutil info / container free space |
| What does the file system report for this volume | df -h / |
| What is macOS willing to promise an app | Finder's "available" |
| Where did the space go, by category | System Settings > General > Storage |
What purgeable actually covers
Purgeable is macOS's running estimate of space it could release under pressure, gathered from several unrelated places:
- Local Time Machine snapshots. Usually the largest contributor, and the one that makes deleted files appear to free nothing. See local snapshots.
- Evictable iCloud Drive and Photos content when Optimize Mac Storage is on. The file is a placeholder locally and the full copy can be re-downloaded.
- Caches macOS considers regenerable, which the system may drop without asking.
- Accounting slack inside the APFS container that is not yet reflected everywhere.
Each of those follows different rules, so "free 47 GB of purgeable space" is a number nobody can verify before the fact. It is a ceiling, and a tool that quotes it as a deliverable is guessing.
You cannot purge it on demand, and that is by design
No supported command says "release purgeable space now." macOS reclaims the blocks when an allocation needs them, which is the useful behaviour: a snapshot or a downloaded cloud file is worth keeping right up to the moment the space is wanted.
So a Mac reporting very little free space can still save a large file or install an update, because the request itself triggers reclamation. To get the space back sooner, remove whatever is holding it instead of hunting for a purge switch. Thinning snapshots is the one lever that reliably moves this number:
tmutil listlocalsnapshots /
tmutil thinlocalsnapshots /
sudo purge is a different thing entirely
Search results conflate the two constantly, so: purge is a real macOS command with
nothing to do with disk purgeable space. It flushes the file system cache in memory,
forcing cached disk pages out of RAM. Apple built it for benchmarking, so a second run of
a test does not read from a warm cache.
sudo purge
It frees no disk space and deletes no files. It throws away cache the system just spent time building, which makes the next few minutes of work slower while everything is re-read from disk. On a healthy Mac there is no reason to run it, and running it on a schedule only costs you performance.
If your actual question is about memory rather than disk, read memory pressure instead: the number that matters there is pressure, not the amount of free RAM.
Why Storage settings takes a while to agree
After you delete something large, the Storage pane often keeps showing the old figure. The categories in that view are computed by a background process that samples the volume rather than recounting it on every change, and several categories overlap.
That makes the pane a good map and a poor instrument. Use it to see the shape of the
volume, and use df or diskutil before and after a change to find out whether the
change worked.
A short diagnostic
When free space does not match your expectation, this order resolves it in a few minutes.
- Record
diskutil info /container free space. That is the honest baseline. - Run
tmutil listlocalsnapshots /. Entries here explain most "deleting freed nothing" reports. - Check whether Optimize Mac Storage is on for Photos and iCloud Drive. If it is, some of your "used" space is evictable and some of your "available" space is theoretical.
- Only then look for large files. If the first three steps explained the gap, you do not have a file problem.
Where a review tool fits
Mole reports what it can prove: real on-disk sizes for the things it lists, before and after totals for anything it removes, and no headline promise about purgeable pools it cannot inventory. When the space is pinned by snapshots rather than by files, the useful answer is to say so rather than to show a bigger number.
FAQ
Should I try to get purgeable space to zero?
No. Purgeable space is mostly working state that benefits you: snapshots you might need, cloud files you can open instantly, caches that keep apps fast. Driving it to zero trades all of that for a number.
Why does Finder show more available space than df?
Finder includes space macOS is confident it can reclaim. df reports what the file
system currently has unallocated. Both are correct answers to different questions.
Does an external drive have purgeable space?
Local snapshots are a startup-volume behaviour, so the largest contributor is usually absent. An external APFS volume can still carry snapshots if something wrote them there.
Is purgeable space a sign of a problem?
Only when it is very large and growing while free space shrinks. That pattern usually means snapshots are accumulating faster than they are being thinned, which the snapshot guide covers.