How macOS actually works

The storage numbers, reconciled

Updated 2026-07-31 · 4 min read

Four tools, four different numbers for the same disk. None is wrong; they measure different things.

The four sources

• Finder (Get Info on the drive) — a quick, cached figure
• System Settings → Storage — categorised, includes purgeable optimistically
• Disk Utility — closer to the truth, shows used/free per volume
• df -h / du — the filesystem's own accounting

They disagree because of four APFS realities.

1. Purgeable space

macOS counts space it *could* reclaim — local snapshots, caches, evictable iCloud files — as available. Storage settings and Finder show optimistic free space; a real large copy may not be able to use it until it is purged.

diskutil info / | grep -i "free\|available"

The two numbers there are the honest gap.

2. Snapshots

Local Time Machine snapshots occupy real space that shows as purgeable:

tmutil listlocalsnapshots /

3. Shared container space

APFS volumes in one container share free space. Each volume reports the container's free space as its own, so the numbers "overlap" — several volumes each claiming the same 200 GB free is correct, not a bug.

4. Clones

A cloned file (an APFS copy) counts its full size logically while occupying nothing extra until modified. Logical sizes overcount real usage.

Which to trust

For "can I actually write 50 GB right now": df -h free space, minus a margin, is the honest floor. For "what is reclaimable": Storage settings' purgeable figure. For "what is physically used": Disk Utility. For "which folder is huge": du.

The practical upshot

When the disk says full but a number says free, it is purgeable space you cannot immediately use. tmutil thinlocalsnapshots / reclaims the fastest chunk of it, and the emergency-space article covers the rest.

Common questions

Why does my Mac show different free space in different places?

Purgeable space, snapshots, shared APFS container space and file clones each skew the count. df -h is the honest floor for what you can write now.

What is purgeable space?

Space macOS can reclaim on demand — snapshots and caches — counted as free but not always immediately usable.