What is purgeable space on a Mac? Why Finder and Settings disagree

What is purgeable space on my Mac?

The short answer

Purgeable space is storage macOS says it can free on its own when something needs room: iCloud files already uploaded, some caches, and Time Machine's local snapshots. Finder counts it as available. The disk doesn't, yet. That's why Finder, System Settings and a download that fails for lack of space can all disagree.

Why it matters

A big app install, a video export or an Xcode build checks the space that's free right now, not the space macOS promises to free later. So you can have "50 GB available" in Finder and still get a disk-full error.

See the real number

This shows free space without purgeable counted in. It only reads:

df -h /System/Volumes/Data

Compare it with what Finder shows (select Macintosh HD and press ⌘I). The gap is purgeable space.

Can I force macOS to free it?

Not directly. It's released when an app asks for space. Local snapshots are listed with tmutil listlocalsnapshots /, and iCloud files can be released by right-clicking them in Finder and choosing Remove Download.