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What is actually taking up space on your Mac

4 August 20269 min read

Ten places the room goes, in the order worth checking. Two of them look enormous and should be left exactly where they are.

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.

There is no folder called System Data. You cannot open it, which is why every guide telling you to "clear System Data" is describing something that does not exist. What you can do is find the real folders underneath the label, and those have ordinary names in ordinary places.

The second complication is purgeable space. Some of what your Mac counts as used is space it will hand back automatically the instant you need it, mostly Time Machine local snapshots and cached iCloud files. This is why your available space sometimes changes while you are watching, and why a 40 GB video file can save successfully on a Mac that claimed to have 12 GB free.

To find space that is genuinely yours to reclaim you need folders sized by what they actually occupy, ranked largest first. That is what a disk map does, and it is the reason this entire 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, and it is completely safe to remove.

~/Library/Developer/Xcode/DerivedData

Delete the whole folder. Xcode rebuilds what it needs on the next build, once, more slowly than usual. Old simulator runtimes are the other half of the problem, and each one is a full copy of iOS:

xcrun simctl delete unavailable

That removes simulators for OS versions you no longer have installed. There is also ~/Library/Developer/Xcode/iOS DeviceSupport, which accumulates a folder for every iOS version of every device you have ever plugged in for debugging. Nothing ever cleans it, and on a Mac used for iOS work over several years it is frequently over 20 GB by itself.

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. Deleting things inside Docker frees space inside the image and changes the file on your disk not at all.

docker system df

That shows the damage before you act, broken down by images, containers, volumes and build cache. 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. Several hundred is not unusual on a family Mac that has been the backup destination for four phones across six years.

Manage them in Finder by selecting your device and choosing Manage Backups, which shows the date and device name for each. Keep the most recent backup for any device you still own. The rest are backups of phones that no longer exist, and deleting them through that interface is safer than removing folders by hand, because the folder names are opaque identifiers that tell you nothing about which device they belong to.

4. Photos

Check "Recently Deleted" inside Photos first. It holds items for 30 days, and on a large library that is routinely several gigabytes of things you already decided you did not want.

If you use iCloud Photos, the setting that matters is Optimise Mac Storage. With it off, every full resolution original lives on your Mac permanently. With it on, smaller versions stay local and originals are fetched when you need them. On a large library this single toggle is often worth more than everything else on this page combined.

The trade-off is real: you need a connection to work with originals. If you edit photos on a plane, leave it off and accept the storage cost.

5. Virtual machines and disk images

Parallels, VMware, UTM and any .dmg you have mounted all create single files that behave the same way Docker does: they expand as the guest writes data and they do not contract when it deletes it.

A Windows virtual machine you set up once to run one application is a 60 GB file that will sit there indefinitely. Most virtualisation software offers a compact or reclaim operation which shrinks the disk to fit its actual contents, and it is worth running before deciding you need to delete the machine entirely.

6. node_modules

Every finished JavaScript project is carrying a few hundred megabytes it will never need again. A single project can hold 250,000 files, none of them large, which makes this invisible to any search that filters by file size. 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 except the reinstall if you ever come back. The same logic applies to .venv, target, build and vendor folders in other ecosystems.

7. Mail

Mail keeps a copy of every attachment you have ever received, whether or not you ever opened it:

~/Library/Mail

Do not delete this folder by hand. Use Mail, then Settings, then Accounts, and set attachments to download recent messages only, then rebuild the mailbox. Deleting the folder directly can take local-only mail with it, meaning anything not stored on the server is simply gone.

This folder is also one of the ones a scan cannot see without Full Disk Access, which is a common reason a disk tool's total comes up short. That mechanism is explained in the article on Full Disk Access.

8. 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, and the installer you used to set up an app two years ago has never once been needed since.

Not glamorous, reliably worth several gigabytes, and the lowest risk cleanup available to anyone.

9. Application caches

~/Library/Caches

Worth a look, rarely worth much, and dramatically over-recommended. Individual apps sometimes hoard gigabytes, browsers and design tools especially, but the folder as a whole is usually smaller than people expect, and macOS refills it within days.

Delete a specific large offender rather than the lot, and quit the app first. Removing a cache underneath a running program can confuse it, because it is holding an open handle to something that no longer exists.

10. 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 single 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. Searching the folder name takes eight seconds and is always cheaper than the alternative.

Move things to the Trash, not past it. Almost every "I deleted the wrong thing" story involves someone who was certain, and certainty is not a backup. The Trash is a free undo that lasts exactly as long as you leave it alone, and emptying it the same day you filled it converts every reversible mistake into a permanent one for no benefit beyond getting your space back a few hours sooner.

Common questions