# Purgeable space on a Mac, and how to force it back

> Purgeable is space macOS believes it can reclaim but has not. What goes into it, why the Finder counts it as available when nothing else does, and the reliable ways to make macOS actually release it.

Canonical: https://bytesweep.app/blog/purgeable-space-mac.html
Last updated: 2026-09-29

## Purgeable space, and how to make macOS actually release it

You delete 30 GB and the Finder’s free space doesn’t move. Disk Utility shows a different number, and the command line a third. None of them is wrong. They answer different questions, and the difference between the answers is purgeable space.

- By [Shajeen Ahamed](https://bytesweep.app/about.html#author)
- Published 29 September 2026
- Last updated 29 September 2026
- 3 min read
- Part of [How to free up space on a Mac](https://bytesweep.app/free-up-space-on-mac.html)
- [How we write](https://bytesweep.app/about.html#how-we-write)

Purgeable space is occupied by things macOS considers expendable when it needs room. It isn’t free, and it isn’t quite used either. The Finder adds it to free space and calls the total “available”, which is optimistic but defensible: if you really needed the room, macOS would release it. Disk Utility and `df` report what’s free right now.

*The Finder’s available figure includes space that is still occupied. df shows only what is free right now.*

### What’s in it

Snapshots, mostly. APFS and Time Machine local snapshots are by far the biggest contributor. A snapshot keeps the old version of every block that has changed since it was taken, which is why deleting a large file while a snapshot exists frees nothing at all.

The rest is local copies of iCloud Drive files that macOS could evict and download again, and caches that apps have registered as discardable through the system’s cache-deletion mechanism. Emptying the Trash turns some of the total into real free space, but it doesn’t touch the snapshots, so it rarely moves the number much.

Swap isn’t part of it. Swap files count as used space, so more RAM means less swap but does nothing for purgeable space.

Purgeable space is also not the grey System Data segment in Storage settings, although the two are often confused. [What is System Data on a Mac](https://bytesweep.app/mac-system-data.html) lists what that segment holds and which parts you can clear.

### See all three numbers

```
df -H /System/Volumes/Data
diskutil info / | grep -i -e "Free Space" -e "Container"
tmutil listlocalsnapshots /
```

`df` gives you the honest figure. `diskutil` reports the container’s free space, which on APFS is shared by every volume in the container. The snapshot list, more often than not, explains the gap.

### Why a clean-up seems to do nothing

Snapshots are point-in-time references to the file system. Delete a file while a snapshot exists and its blocks are still referenced, so nothing comes back until the snapshot expires (24 hours later by default) or is deleted.

So on a Mac with local snapshots, cleaning up doesn’t free space until the snapshots roll off. People conclude the clean-up failed, repeat it, and delete far more than they meant to. [How local snapshots work and how to thin them](https://bytesweep.app/blog/time-machine-local-snapshots.html) has the commands.

### Making macOS let go

Create real pressure. This is what the mechanism is designed for. Write a large file, then delete it:

```
mkfile 30g ~/ballast
rm ~/ballast
```

macOS sees the demand, purges snapshots and caches to meet it, and the space is free afterwards. `mkfile` writes every byte, so this takes a few minutes, and the size should stay below what the Finder calls available so you don’t fill the disk. Don’t add `-n`: that flag only notes the size without allocating any blocks, so it creates no pressure at all.

Or thin the snapshots directly, which is more targeted:

```
tmutil thinlocalsnapshots / 21474836480 4
```

That asks for 20 GB back at urgency level 4, the highest. If Time Machine is your only backup, don’t delete every snapshot out of habit: the newest one is the local record of changes not yet copied to the backup drive.

Or restart. It’s unfashionable and it works: swap files and a lot of cached state go, and startup is a natural moment for the system to reconcile purgeable space.

Tools that claim to “delete purgeable space” are almost always doing the first of these, writing a large file and removing it. Nothing else is available to them.

### When it won’t budge

Occasionally the figure stays put through all of that. Two causes are worth knowing.

A running backup or a cloning tool may be holding a snapshot. `tmutil listlocalsnapshots /` shows Time Machine’s; the backup has to finish or be cancelled.

Or another volume in the same APFS container has reserved space: a second macOS installation, a volume left over from a Boot Camp-era setup, or one left behind by a failed upgrade. `diskutil apfs list` shows every volume sharing the container, and the recovery partition and other invisible volumes explains how to read that output.

Under pressure, purgeable space really is available, and macOS is reliable about releasing it. A 40 GB download will succeed. What you can’t do is plan around it. When the Finder says 60 GB available and `df` says 8 GB, the 8 is the number to work with.

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Related in Bytesweep: [Junk cleaner](https://bytesweep.app/junk-cleaner.html)

### Keep reading

- [**Time Machine keeps snapshots on your Mac, not just the backup drive** macOS takes hourly snapshots of your internal disk whether or not a backup drive is attached, and they commonly hold 10 to 80 GB. How to list them, how to thin them, and which one you must not delete.](https://bytesweep.app/blog/time-machine-local-snapshots.html)
- [**APFS snapshots: the space you can’t see or easily delete** A snapshot is a frozen reference to every block on the volume at one moment. What that means for free space, why a snapshot has no single size, and how to read diskutil's snapshot list properly.](https://bytesweep.app/blog/apfs-snapshots-explained.html)
- [**You emptied the Trash and got nothing back: here is why** Four separate mechanisms stop a deleted file from returning its space on macOS, and only one of them is the Trash. How to tell which one you have hit, and what to do about each.](https://bytesweep.app/blog/empty-trash-mac-space.html)
- [**“Your startup disk is almost full” on a Mac: what to do** The macOS warning appears only when a few gigabytes are left, which is late. Here is the order to work in, the four folders that are usually the whole answer, and what to do if the disk is already too full to delete anything.](https://bytesweep.app/blog/mac-startup-disk-full.html)