What is purgeable space on a Mac?

Purgeable space is storage that macOS says it can free by itself when something needs it. Apple describes it as space macOS makes available by removing files when needed, files you can't remove yourself. It's why a Mac can report a lot of available space and still refuse a large copy.

What makes space purgeable

• Time Machine's local snapshots. Snapshots hold space for files you've changed or deleted. macOS removes them as they age, or sooner when space runs low.
• iCloud files stored on your Mac. With Optimize Mac Storage on for iCloud Drive, macOS can remove the downloaded copy of older files and keep them in iCloud, downloading them again when you open them.
• Files apps mark as purgeable: copies macOS can download again or do without, such as some caches and downloaded media.

What's purgeable isn't wasted, and it isn't junk: it's space in use that macOS has agreed to give up when asked.

Where you see it

• Finder counts purgeable space as available. That's the figure most people see first.
• Disk Utility shows it separately. Select your startup disk's volume and click the Info button in the toolbar to see the used, purgeable and free space.
• Storage settings doesn't use the word. It shows what's used and available, and the snapshots behind much of the purgeable space often end up inside System Data.

Why a large copy can still fail

macOS frees purgeable space when something asks for it, but not always as fast as the request arrives. A big download, a video export or a macOS update can check for room, find too little truly free space, and stop, while Finder still says there's plenty.

If that happens:

1. Try again after a minute or two; macOS may have freed space in the meantime.
2. Remove Time Machine's local snapshots now, instead of waiting for macOS. See Time Machine local snapshots.
3. For iCloud Drive files, Control-click those you don't need on this Mac in Finder and choose Remove Download. They stay in iCloud and on your other devices.

How the app does it

The app's space meter shows the available space beside a bar that ends in what's truly free, and its Why the numbers differ panel says how much of the difference is purgeable and how many local snapshots there are, so you can see how much of "available" is really free before you start something big. The menu bar item shows the available space all day, and can check whether a waiting macOS update fits.

When snapshots hold the space, its Local Snapshots page lists them and can remove Time Machine's with Time Machine's own tool, measuring what came back. iCloud Drive files are never removed by the app: their catalog entry explains Remove Download instead.

The space meter is free. Removing snapshots is PRO.