Reading the Mac storage bar, category by category
The coloured bar in Storage settings is a summary, not a map. Here is what each segment counts, and why Documents is usually the misleading one.
System Settings, General, Storage gives you a bar in eight or nine colours and a list beside it. It is the first place everyone looks, and it answers a slightly different question from the one being asked. The bar tells you which category macOS has filed your space under. It does not tell you which files.
What each segment actually counts
| Category | What goes in it |
|---|---|
| Applications | Everything in the Applications folder, but not the support files an app leaves elsewhere |
| Documents | A catch-all for large files anywhere in your home folder, whatever the type |
| iOS Files | Backups of iPhones and iPads made on this Mac |
| Photos | The Photos library, including originals not yet offloaded to iCloud |
| Downloaded messages and attachments held locally | |
| Music Creation | Instrument and loop libraries, which run to tens of gigabytes if they were ever installed |
| Trash | Items you have removed but not yet emptied |
| macOS | The operating system itself |
| System Data | Everything the categories above did not claim |
Documents is the segment that misleads
Most people read Documents as the Documents folder. It is not. It is a size based catch-all: large files sitting anywhere in your home folder get counted here, including downloads, disk images, exports and archives that have nothing to do with documents in any ordinary sense.
This is why the number is so often the largest one on the list and so rarely leads anywhere. Opening the Documents folder and finding four gigabytes when the bar says sixty is not a bug: the other fifty six are somewhere else in your home folder.
System Data is the other one
System Data is defined by what it is not. Every file macOS could not file elsewhere lands there, which is why it grows without any obvious cause and why no amount of staring at the bar explains it. It has its own guide: what System Data actually is.
Why the total does not match
Three separate effects make the bar disagree with other numbers on the same Mac:
- Free space in Finder counts space that can be reclaimed, so it is often more generous than the bar. See purgeable space.
- The bar is recalculated lazily. Removing a large folder can take minutes to show, and sometimes needs the panel closed and reopened.
- Disk Utility counts a whole container rather than the volume you store files on, which is a different number again: other volumes in container.
What to do with what it tells you
Treat the bar as a signpost. A large Documents number means look through your whole home folder by size rather than in the Documents folder. A large System Data number means look at caches and snapshots. A large Applications number is the one case where the obvious action, uninstalling something, is also the right one, though most uninstalls leave files behind.
Common questions
What does Documents mean in Mac storage settings?
It is a size based catch-all rather than the Documents folder. Large files anywhere in your home folder are counted there, including downloads, disk images and exports, which is why the number is often far larger than the Documents folder itself.
Why does the Mac storage bar not match the free space in Finder?
Finder counts space macOS can reclaim on demand, mainly local snapshots and caches, while the storage bar shows it as used. The bar is also recalculated lazily, so a large removal can take several minutes to appear.
What is System Data in the storage bar?
It is the remainder: every file that none of the named categories claimed. That is why it grows for no visible reason and why it cannot be opened from the panel. Caches, logs, snapshots and support files all land in it.