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

What macOS counts as System Data

The category used to be called Other. It is a remainder: macOS adds up what it can classify, and whatever is left on the disk lands here. In practice that means these things.

• Caches: files apps and macOS can rebuild, mostly in ~/Library/Caches and /Library/Caches.
• Logs and diagnostic reports, in ~/Library/Logs and /private/var/log.
• Swap files and the sleep image, in /private/var/vm. macOS uses them for memory.
• Local Time Machine snapshots: hourly copies kept on the startup disk itself.
• Developer data: Xcode build folders, simulators, package manager caches, Docker disk images.
• App support files: databases, downloads and settings in ~/Library/Application Support and ~/Library/Containers.
• System support volumes: the hidden Preboot, Recovery, VM and Update volumes that sit beside your data.

Why the number is so large, and why it keeps changing

System Data is large because the things inside it grow quietly. A browser keeps a cache for every site you visit. A chat app keeps every image you scrolled past. Xcode keeps build output for every project you ever opened. None of it shows up in your Documents folder, so you never see it.

It changes from hour to hour because parts of it are temporary. Swap grows when memory is under pressure and shrinks after a restart. Snapshots appear every hour while Time Machine is turned on and expire after about a day. Caches are trimmed by macOS when space runs low. A figure that moves by several GB in a day is normal.

The Storage screen also takes a while to calculate. If the number looks wrong just after you deleted something, leave the window open for a minute.

How to see how much System Data you have

Open the Apple menu, choose System Settings, then General, then Storage. Wait for the bar to finish calculating. System Data is listed near the bottom, next to macOS.

Most categories have an info button that opens a list of files. System Data does not, which is why it feels opaque. To see what is behind it, look at your Library folder, where most of it lives. This command lists the largest folders in your user Library and deletes nothing.

du -sh ~/Library/* 2>/dev/null | sort -h | tail -n 15

What is safe to clear by hand

Caches and logs are the safe part. Apps rebuild caches when they need them, and logs are only useful when you are diagnosing a problem. Quit the app first, so it is not writing to the files you are removing.

The command below shows which caches are biggest, so you can decide what is worth clearing. It only reads sizes.

• In Finder, choose Go, then Go to Folder, and enter ~/Library/Caches.
• Sort by size, or use the command below to find the largest folders.
• Move the contents of the large folders to the Trash. Leave the top level folders themselves in place.
• Do the same in ~/Library/Logs if it is large.
• Restart the Mac, check that your apps open normally, then empty the Trash.

du -sh ~/Library/Caches/* 2>/dev/null | sort -h | tail -n 15

Local snapshots and developer data

If you use Time Machine, macOS keeps local snapshots on the startup disk so you can restore recent versions without the backup drive. They expire on their own after about 24 hours, and macOS deletes them earlier when it needs the room. You rarely have to touch them. To see whether any exist, run the command below.

If you need the space immediately, you can delete one with sudo tmutil deletelocalsnapshots followed by the date part of its name, for example 2026-10-10-093012. That removes only that snapshot, not your backups on the Time Machine drive.

If you write software, developer data is often the largest part of System Data. Xcode's DerivedData folder, old simulators, node_modules folders and package manager caches can reach tens of GB together. Build output and download caches are safe to remove: the cost is a slower next build.

tmutil listlocalsnapshots /

What you should leave alone

Some of System Data belongs to macOS, and deleting it by hand causes problems or does nothing useful.

• Swap files and the sleep image in /private/var/vm. macOS creates and removes them. A restart clears swap.
• Anything in /System, and the Preboot, Recovery and Update volumes. macOS needs them to start and to update.
• ~/Library/Application Support as a whole. It holds real data: mail, notes, app databases, game saves, iPhone backups. Remove a folder only if you know which app owns it and you no longer use that app.
• ~/Library/Containers and ~/Library/Group Containers as a whole, for the same reason.
• Folders in /private/var/folders. They are temporary files macOS clears itself.

Where Diskzy fits

Diskzy is a native Mac app that maps your whole disk. Its System Data view breaks the single grey figure into named parts: developer data, caches, app data, logs, swap and sleep image, macOS support files, backups, AI models and more. Each part shows its size and one of three labels: Safe to clear, Look first, or Managed by macOS.

That view is for understanding. Deleting happens in the Clean up tab, where each item is ticked or unticked with a reason, and apps that are open are left alone. Everything Diskzy removes goes to the Trash, so nothing is gone until you empty it. It is free for 72 hours with everything unlocked, then $9 once for 2 Macs.