What is System Data on Mac? What is in it, and how to shrink it

System Data is the part of your Mac's storage that macOS cannot put under a named heading such as Applications or Photos: caches, logs, temporary files, local Time Machine snapshots, swap, and app data in your Library. Some of it is yours to remove, and some of it macOS will only let go of itself.

Why it looks so big

System Settings → General → Storage sorts your disk into Applications, Documents, Photos, Mail and a few more, and whatever it cannot attribute lands in System Data. It is not one folder, so there is no single thing to delete, and the figure includes space macOS is already willing to give back on demand. On macOS Big Sur and earlier the same bucket was called Other; see where Other storage went.

That last part matters. APFS reports some space as purgeable — snapshots and caches the system will drop by itself when an app asks for room — and it is counted in System Data even though it is not really occupied. The number can fall by gigabytes without you doing anything.

What is usually in it

Caches in ~/Library/Caches, and in the hidden ~/.cache that command-line tools share. See clearing the cache on a Mac.
Logs in ~/Library/Logs, crash reports included.
Temporary files in /private/var/folders, which macOS owns and running apps hold open.
Local Time Machine snapshots, kept on the internal disk between backups.
Swap and the sleep image, on the separate VM volume at /System/Volumes/VM.
Developer and AI tool data: Xcode build trees, simulator runtimes, package caches, and the models and runtimes AI tools download. See AI tool data on a Mac.

Measuring it yourself

du -sh ~/Library/Caches ~/Library/Logs ~/.cache 2>/dev/null
tmutil listlocalsnapshots /
sysctl vm.swapusage

The first line sizes the parts you own. The second lists local snapshots; the third shows how much swap is in use right now. Swap shrinks on its own as memory pressure falls, and a restart clears it.

Run on the Mac this app was built on, on 1 October 2026, with 8.7 GB free of 245.1 GB: caches and logs together came to about 2 GB, there were 3 local snapshots — every one made for a macOS update — and 1.09 GB of swap was in use. None of those is where a full disk's missing gigabytes usually are; on that Mac they were in developer and AI tool folders, which Storage settings also files under System Data.

What you can remove, and what you cannot

Caches and logs: yes. They regenerate. Quit the apps first so they are not rewriting them while you delete.
Temporary files: restart instead. Deleting from /private/var/folders while apps run can crash them; macOS clears most of it at startup.
Snapshots: only the purgeable ones. Snapshots made for an OS update are marked not purgeable and stay whatever you ask. On the Mac this app was built on, all three local snapshots were of that kind, so thinning them would have freed nothing.
App data in your Library: carefully. ~/Library/Application Support holds caches for some apps and the only copy of somebody's work for others, and the folder names rarely say which.

Where the app comes in

RefreshYourMac measures each of those parts as its own category, so System Data stops being one unexplained number. Caches and logs are rated Safe to Remove; temporary files are shown with the advice to restart; Application Support is shown for its size and never deleted. Its maintenance jobs count how many local snapshots are actually purgeable before offering to thin them, and say so when the answer is none.