What is taking up space on your Mac? The complete map
Your Mac's storage bar is full but you can't find why? Here's the complete map of where Mac storage actually goes (System Data, caches, snapshots, and the apps that quietly hoard gigabytes) and where to look for each.
Your storage bar is full, you've deleted files, and it still won't budge. The reason is that most of a Mac's used space isn't the files you can see; it's caches, System Data, APFS snapshots, and app libraries that macOS never shows you plainly. This is the complete map of where it goes, and the guide for each piece.
To see the official breakdown first: Apple menu → System Settings → General → Storage. That colored bar is the starting point, but its biggest, vaguest segment ("System Data") is exactly the part it won't explain. That's what most of this map is about.
The storage categories macOS won't explain
- What is "System Data" on Mac (and how to shrink it): the big catch-all segment, decoded.
- What is "Other" storage on Mac?: the old name for the same thing, and what changed.
- Why "Documents" takes up so much space: why that slice is broader than your Documents folder.
- What is purgeable space on Mac?: the space macOS frees automatically when it needs room.
- The hidden storage on your Mac: where your disk space really goes: the full tour of the 100–200 GB you never chose to keep.
Why the numbers don't add up
If deleting things doesn't free space, or the totals seem wrong, the cause is usually one of these:
- Why is my Mac storage full when I deleted everything?: APFS snapshots, purgeable space, and stale counts.
- You deleted files but free space didn't change: here's the fix: the Trash, snapshots, and Finder's lag.
- Why your Mac has two "Macintosh HD" volumes: the read-only system volume vs your data volume.
- Time Machine local snapshots: the hidden GB behind "System Data": backups sitting on your internal disk.
The apps that quietly hoard gigabytes
Specific apps are often the real answer. If one of these is on your Mac, it's probably a big chunk of your storage:
- Photos eating your storage? iCloud "Optimize" explained
- Why Apple Mail is taking up so much space
- GarageBand's sound library: is it safe to delete?
- Why Xcode takes up 50+ GB (and what's safe to delete)
- Android Studio eating 50–100 GB? What to clear
When it's urgent
If you're already out of room and something's failing:
- "Your startup disk is almost full": what to actually do
- "Not enough space to install the update": the fix
Then: free it, the honest way
Once you know what's taking the space, the companion guide covers reclaiming it safely, starting with Apple's own tools before any third-party app:
See the hidden space named, sized, and yours to clear
The reason this is hard by hand is that the biggest categories are the ones macOS keeps vague. Plumby decodes "System Data" into what it actually is (Docker images, snapshots, caches, dev junk), each named with its exact path, sized, and clearable only on your command. It counts the bytes it actually removes, so the number you get back is real, and it never touches your documents, photos, or projects.
