What is System Data on a Mac, and how do you reduce it?

Open System Settings ▸ General ▸ Storage and one of the biggest bars is often System Data, sometimes 50 or 100 GB. Unlike the other categories, it has no ⓘ button, so Storage won't tell you what's in it. Here is what it usually is, and how to find the part you can remove.

What System Data is

System Data is what's left over: everything Storage can't put in one of its other categories. It used to be called “Other”. Apple describes it as files the system uses, such as logs, caches, virtual memory, and temporary files. In practice it also includes a lot that belongs to you:

• Local Time Machine snapshots. If you use Time Machine, macOS keeps snapshots on the internal disk too, so you can restore files while the backup disk is away.
• App data in ~/Library: caches, Application Support and app containers. This is where Docker's disk image, Xcode's simulators and build folders, and many apps' downloaded content live.
• Virtual machines kept inside apps' containers.
• iPhone and iPad backups made with the Finder.
• Swap and the sleep image in /private/var/vm, which macOS resizes itself.

Some of it is the system's own and should stay. The big, surprising numbers almost always come from the first two: snapshots and app data.

Step 1: restart, and give it a minute

A restart clears swap files and temporary files, and macOS often cleans up caches afterwards. It takes a minute and removes nothing you need, so do it before measuring anything. Storage can take a few minutes to recalculate after that.

Step 2: local Time Machine snapshots

macOS deletes local snapshots on its own when space runs low, so they rarely cause a disk to fill up. They do make System Data look huge in the meantime. To see them, open Terminal and run:

tmutil listlocalsnapshots /

Each line is a snapshot with a date. To delete one, pass its date:

tmutil deletelocalsnapshots 2026-09-20-101530

Your Time Machine backups on the backup disk are separate, and deleting local snapshots doesn't touch them.

Step 3: find the folders behind it

Everything else in System Data is ordinary files in ordinary folders, mostly in ~/Library, /Library and /private/var. Storage won't show them, but a disk tool will, once it's allowed to read them. First give it Full Disk Access in System Settings ▸ Privacy & Security ▸ Full Disk Access. Without it, macOS hides other apps' containers, which is often exactly where the space is.

In the Terminal, this lists what's in ~/Library, largest last:

du -h -d 1 ~/Library 2>/dev/null | sort -h

A treemap is quicker to read. Scan your home folder in Layland and ~/Library shows up as one area of the map, with its biggest folders visible straight away. With Show Free Space on, the rest of the disk appears too, so you can see how much of the disk your home folder accounts for, and how much is outside it.

The usual culprits, and how to clear them

Use each app's own cleanup where there is one. It knows what's safe to remove.

• Docker Desktop keeps all images and containers in one disk image in ~/Library/Containers/com.docker.docker. Run docker system prune to remove stopped containers and unused images, or lower the disk limit in Docker's settings.
• Xcode: in ~/Library/Developer, Xcode/DerivedData is build output and safe to delete; Xcode rebuilds it. xcrun simctl delete unavailable removes simulators for versions you no longer have, and Xcode/iOS DeviceSupport has a folder for every iOS version you've connected a device with.
• iPhone and iPad backups: connect a device, select it in the Finder's sidebar, click Manage Backups, and delete the backups of devices you no longer have. They're in ~/Library/Application Support/MobileSync/Backup.
• Caches in ~/Library/Caches: safe to remove, since apps rebuild them, but they grow back. Quit the app first. Homebrew's is cleared with brew cleanup.
• Virtual machines: delete the ones you don't use from inside Parallels, UTM or VMware.

Leave /System, /private/var/db and anything you can't identify alone, and move files to the Trash instead of using rm, so you can put them back.

Storage may keep showing the old number for a while after you delete things. It recalculates in the background; a restart gets it there sooner.

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