Where you see it
Open System Settings › General › Storage and hold the pointer over the bar at the top. Next to Applications, Documents, Photos and macOS there's a grey part called System Data. On many Macs it's the biggest part of the bar, and unlike the others you can't click into it to see what it is.
What's in it
System Data is everything Storage settings doesn't put in another category. On most Macs that means a mix of these:
- App data in your Library folder: settings, databases, offline files and downloads apps keep for you. Often the biggest single part.
- macOS startup and recovery: hidden volumes macOS needs to start up, update and run Recovery, and updates waiting to install.
- Other system files: shared app support, developer tools in /opt and /usr/local, and system databases.
- Time Machine local snapshots: copies of your disk kept on the Mac itself while Time Machine is on, and around macOS updates.
- The Spotlight index: what makes search fast. It can swell after a migration or an indexing problem.
- Swap files and the sleep image: memory moved to disk. A restart clears the swap.
- Apple Intelligence and other Apple downloads: on-device models, voices and dictation assets.
- Aerial screen saver and wallpaper videos you have downloaded.
- iPhone and iPad backups made on this Mac.
- Xcode and developer files: simulators, build folders, device support.
- App caches and logs, plus container and virtual machine disks, and more.
The causes people usually talk about (snapshots, Spotlight, swap, backups, caches) are only part of the story. On our own test Mac they made up about a third of System Data; app data, the startup and recovery volumes and other system files made up most of the rest.
Apple calculates the System Data figure as an estimate, partly by subtracting the other categories from the space in use. That's why it can change between one look and the next.
Why it can be 100 GB, 200 GB or more
A big System Data number often comes down to one or two causes that grew. Apps keeping a lot of data in your Library, a Spotlight index that ballooned, years of iPhone backups, developer tools, or local snapshots that linger can each account for much of it. The number alone doesn't tell you which. The common advice to wait overnight works only for the parts macOS cleans up by itself, and erasing the Mac fixes it by removing everything.
How to check it yourself
- Restart the Mac. This clears swap files and lets macOS finish any clean-up it was in the middle of. Then look at Storage settings again after a few minutes.
- Look for local snapshots. Open Terminal (in Applications › Utilities) and run the command below. Each line is one snapshot. Our snapshot guide shows how to delete them.
tmutil listlocalsnapshots / - Check iPhone and iPad backups. Connect the device, select it in the Finder sidebar and click Manage Backups…. Old devices' backups can go.
- Look at the caches. In the Finder choose Go › Go to Folder…, type
~/Library/Cachesand press Return. Choose View › Show View Options, turn on Calculate all sizes, and sort by size. A cache of an app you no longer use can go to the Trash. The space is only free once you empty the Trash. - Measure the Spotlight index. It sits in a protected folder, so the command needs your password:
If it's much larger than you'd expect, see why the Spotlight index gets so big.sudo du -sh /System/Volumes/Data/.Spotlight-V100 - Check the rest: your Library folder (
~/Library/Application Supportand~/Library/Containers, sorted by size the same way), Apple Intelligence models, aerial videos and, if you develop apps,~/Library/Developer. Remove an app's data from inside the app rather than by deleting its folder.
What not to do
- Don't delete folders in /System or /private by hand, or use rm in Terminal on anything you aren't sure about. Some of System Data is what keeps macOS running.
- Don't turn off Spotlight for good to save space: Mail, Finder search and many apps rely on it.
- Make sure you have a backup before a big clean-up.