Free, available and purgeable
Three words get mixed up here, and Apple uses them precisely:
- Free space is space nothing is using.
- Purgeable space is used by files macOS has marked as removable. Apple says macOS can free it when needed by removing those files, and that you can't remove purgeable files yourself (Apple, Get information about a disk in Disk Utility).
- Available space is free space plus purgeable space. Apple puts it as "available storage space includes free storage space and purgeable disk caches", and notes that space can be both available and used at the same time (Apple, Find and delete files on your Mac).
So when Storage settings or Finder says you have 40 GB available, some of that may be purgeable: it holds data right now, and macOS will clear it the moment something needs the room.
What counts as purgeable
Apple doesn't publish a complete list, but its documentation names these:
- Time Machine local snapshots. Apple says your Mac counts the space they use as available storage, and that they don't use space needed for downloading, copying or installing (Apple, About Time Machine local snapshots).
- Caches and logs that are safe to delete, temporary database files, interrupted downloads, staged macOS and app updates, and Safari website data. macOS clears these when space is needed (Apple, Optimize storage space).
- Copies of files that are also in iCloud, if you use Store in iCloud: when space is needed, only recently opened files stay on your Mac, and the rest download again when you open them.
- Apple TV movies and shows you've watched and older Mail attachments, if Optimize Storage is on.
You don't decide what's purgeable. macOS and the apps that store these files do.
Why purgeable space doesn't free up when you expect it to
Because macOS only clears it when something actually needs the space. It doesn't empty snapshots and caches in advance just because the disk is getting full, and it won't clear them because you looked at the number. That's by design: snapshots let you restore files from earlier today, and caches make apps faster.
This usually works without you noticing. It causes trouble in two cases:
- An app checks free space instead of available space. Some apps and installers measure space their own way and may refuse to start because they see only the free part. Ordinary downloads and copies aren't affected: Apple says snapshot space isn't held back from downloading, copying or installing.
- The number looks wrong. If Storage settings shows a large purgeable or System Data figure, it's tempting to think something is broken. Usually it isn't: it's snapshots and caches waiting to be cleared.
How to free it now if you need to
- Snapshots: these are usually the biggest part. You can delete them with
tmutilin Terminal, or by setting Time Machine to back up manually for a few minutes. Here's how, and what you lose. - iCloud files: in Finder, Control-click files or folders in iCloud Drive and choose Remove Download. They stay in iCloud and download again when you open them (Apple, Work with folders and files in iCloud Drive).
- System caches: Apple says starting up in safe mode clears certain system caches, which can free enough space temporarily, for example to install a macOS update (Apple, Free up storage space on Mac). Restart normally afterwards.
If what you really need is more free space overall, purgeable space is the wrong place to start: it comes back as soon as snapshots and caches build up again. Freeing up space properly means finding what's big and removing what you don't need.
What "Other Volumes in Container" means
Older versions of macOS showed an "Other Volumes in Container" figure in their storage view, and it still comes up in forum threads. Apple has no current support article that uses the name, and it isn't a category on the macOS 15, 26 or 27 Storage pages.
What it referred to is real, though. Your Mac's internal disk is an APFS container that holds several volumes sharing the same free space. Apple says a startup disk on macOS 10.15 and later has at least five: System (macOS itself), Data (your files and apps), and three hidden ones, Preboot, VM (for swap files) and Recovery (Apple Platform Security, Role of Apple File System). Space used by volumes other than the one you're looking at is "other volumes in the container".
On our test Mac, a MacBook with macOS 27.0.1, diskutil apfs list (read-only) showed the Preboot volume using 10.9 GB, Recovery 1.5 GB and the macOS System volume 13.7 GB. None of that is yours to delete. To see the same picture without Terminal, open System Settings › General › Storage and click All Volumes. Apple's guide shows space used by other volumes in the container as a crosshatched area in Storage (Apple, See if your Mac shares space across APFS volumes).
Why apps report different free space
Ask four tools how much space you have and you can get four answers. On our test Mac, at the same moment:
| Where | Showed | What it counts |
|---|---|---|
df -h in Terminal | 30Gi | Free space only, in binary units (1 Gi = 1,073,741,824 bytes) |
diskutil info / | 32.0 GB container free space | Free space only, in decimal units (1 GB = 1,000,000,000 bytes) |
| macOS's "available for important use" figure | 37.8 GB | Free space plus what macOS expects to clear by purging caches and other non-essential data |
The first two are the same amount of space in different units; Apple explains the two ways of counting in How storage capacity is measured on Apple devices. The third is the figure apps get when they ask macOS how much room there is for something you've asked for, such as a download. The description of this value in Apple's Foundation framework, which apps are built with, says it includes "space expected to be cleared by purging non-essential and cached resources". The roughly 6 GB gap was purgeable space.
Disk analyzer apps add another layer: they add up the files they can see, and most can't see everything (other volumes, snapshots, other users' files, protected folders). Their total won't match Storage settings, and that's normal. How the main disk analyzers compare.
How Tend helps
Tend's storage view and Space Map show what's taking space in your home folder, and its Maintenance page lists your Time Machine local snapshots, both free. That makes it easy to tell whether your missing space is files you can deal with or snapshots macOS will clear on its own. Tend can't change how macOS counts purgeable space, and it doesn't touch the other volumes in your disk's container.
Questions
What is purgeable space on a Mac?
It's space used by files macOS can remove by itself when it needs room, such as Time Machine local snapshots, caches and copies of files that are also in iCloud. macOS counts it as available, together with free space.
How do I clear purgeable space on a Mac?
Usually you don't need to: macOS clears it when space is needed. If an app insists on more free space now, delete Time Machine local snapshots, choose Remove Download for iCloud Drive files you don't need locally, or restart in safe mode to clear certain system caches. Apple says you can't remove purgeable files directly.
What is "Other Volumes in Container"?
It's a label from older versions of macOS for space used by the other APFS volumes on your disk, such as the macOS System, Preboot, VM and Recovery volumes. They share free space with your Data volume, and they're managed by macOS. Current versions show the same information under Storage › All Volumes.
Why does Disk Utility show different free space from Finder?
Tools count different things. Some show only free space, while Finder and Storage settings show available space, which includes purgeable space macOS can clear. Terminal's df also uses binary units (Gi), which make the same amount of space look smaller than decimal gigabytes.
Related guides
How to delete Time Machine local snapshots
The biggest part of purgeable space, and how to remove it when you must.
How to free up space on your Mac, safely
Find what's big and clear it in the right order.
What is System Data on Mac?
macOS's catch-all storage category, explained.
DaisyDisk vs OmniDiskSweeper vs GrandPerspective
Why each disk analyzer shows a different total.
Sources
- Apple, Get detailed information about a disk in Disk Utility on Mac
- Apple, Find and delete files on your Mac
- Apple, About Time Machine local snapshots
- Apple, Optimize storage space on your Mac
- Apple, Free up storage space on Mac
- Apple, Work with folders and files in iCloud Drive on Mac
- Apple Platform Security, Role of Apple File System
- Apple, See if your Mac shares space across APFS volumes
- Apple, How storage capacity is measured on Apple devices
- Apple Developer, volumeAvailableCapacityForImportantUsageKey (and its description in the macOS SDK's Foundation headers)
See what's really using your space
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