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What is actually taking up space on your Mac

30 August 20269 min read

Nine places the room goes, in the order worth checking. Two of them look enormous and should be left alone.

Why "About This Mac" is not enough

macOS gives you a coloured bar and a category called System Data that is routinely 100 GB and tells you nothing about what is inside it. That category is not a single thing. It is caches, logs, sleep images, iOS backups, Mail attachments, virtual machine disks and whatever else did not fit the other labels.

To find real space you need to see folders sized by what they occupy, ranked. That is what a disk map does, and it is the reason this whole category of app exists.

1. Xcode, if you have it

Derived data is build output. It regenerates. It is also usually the biggest single folder on a developer's Mac.

~/Library/Developer/Xcode/DerivedData

Delete the whole folder. Xcode rebuilds what it needs on the next build, slower once. Old simulator runtimes are the other half:

xcrun simctl delete unavailable

2. Docker

Docker keeps every image layer, container and volume you have ever pulled, inside one disk image that grows and does not shrink on its own.

docker system df

That shows you the damage before you act. To reclaim:

docker system prune -a --volumes

3. Old iPhone and iPad backups

Every device you have ever plugged in, at every OS version, sitting here:

~/Library/Application Support/MobileSync/Backup

Fifty gigabytes is common. Manage them in Finder → your device → Manage Backups, which shows the date of each. Keep the most recent for any device you still own; the rest are dead weight.

4. Photos

Check "Recently Deleted" inside Photos first: it holds items for 30 days and people are routinely surprised by what is in there. If you use iCloud Photos, turning on "Optimise Mac Storage" moves originals to iCloud and keeps smaller local versions.

5. node_modules

Every finished JavaScript project is carrying a few hundred megabytes it will never need again. Find them all:

find ~ -name node_modules -type d -prune -print0 | xargs -0 du -sh | sort -hr | head -20

They rebuild from package.json with one command, so deleting them from dormant projects costs nothing but the reinstall if you return.

6. Mail

Mail keeps a copy of every attachment you have ever received:

~/Library/Mail

Do not delete this folder by hand. Use Mail → Settings → Accounts and set attachments to download recent messages only, then rebuild the mailbox. Deleting it directly can take your local-only mail with it.

7. Downloads

Disk images, installers, and the same PDF four times. Sort by size in Finder and be ruthless: almost everything here can be downloaded again.

8. Application caches

~/Library/Caches

Worth a look, rarely worth much. Individual apps sometimes hoard gigabytes: browsers, Spotify, design tools, but the folder as a whole is usually smaller than people expect, and macOS refills it. Delete a specific large offender, not the lot.

9. Time Machine local snapshots: leave these alone

macOS keeps local snapshots on your startup disk between backups. They can occupy tens of gigabytes and they are the most common thing people try to delete after seeing them in a disk map.

The two rules worth keeping

If you do not recognise it, do not delete it. A folder with a reverse domain name belongs to an app. If that app is still installed, its cache is doing a job.

Move things to the Trash, not past it. Almost every "I deleted the wrong thing" story ends differently when the file is still recoverable. It costs nothing to empty the Trash a week later once nothing has broken.