What is System Data on a Mac, and can you clear it?

System Data is the catch-all category in your Mac's storage settings: caches, logs, temporary files, Time Machine local snapshots, virtual memory and anything else that doesn't belong to Applications, Documents, Photos or the other named categories. Much of it is managed by macOS and shrinks on its own, so the safest ways to reduce it are a restart, a finished backup and a little patience, not deleting system folders.

It still worries people, because it can be huge and you can't click into it to see what is inside. You can find out most of what is in there, though, and clear the parts that are genuinely safe.

Where to find System Data

Open System Settings > General > Storage. The bar at the top splits your disk into colored sections, and the list below gives each category a size: Applications, Documents, Photos, iOS Files, macOS, System Data and so on. On a full disk the numbers can take a minute or two to settle while macOS counts.

Most categories have an info button that opens a list of files you can sort and delete. System Data usually doesn't, which is part of why it feels mysterious. Older versions of macOS showed much of the same space under a category called Other.

Two things make the figure jumpy. It is partly a remainder, whatever is left after the other categories are counted, so anything macOS can't classify ends up there. And some of it is purgeable, meaning macOS can remove it by itself when it needs room. Look twice in one afternoon and you may see two different numbers.

What is in System Data?

The exact mix differs from Mac to Mac. These are the usual ingredients, where they live, and whether to touch them.

To look at your own Library folders, open Finder, choose Go > Go to Folder (Shift, Command and G), and type a path such as ~/Library/Caches. Switch to list view, choose View > Show View Options, and tick "Calculate all sizes" to see which folders are large.

Why is System Data so large?

A few things push it up, and most of them are temporary.

• Time Machine local snapshots. If you use Time Machine, macOS keeps snapshots of your startup disk on the disk itself, so you can restore files even when the backup drive isn't connected. Apple says hourly snapshots are kept for about 24 hours, plus one of your last successful backup until the space is needed. If your backup drive hasn't been connected for a while, they can add up.
• Swap. When apps need more memory than the Mac has, macOS moves some of it to swap files on the startup disk. After a long stretch of heavy work and no restart, these can reach many gigabytes. In Activity Monitor's Memory tab, the Swap Used figure at the bottom shows how much is in use right now.
• Caches that never get trimmed. Browsers, music and video streaming apps, and messaging apps can all keep large caches.
• Developer tools and virtual machines. Build caches, simulators and virtual machine disk images are among the biggest files on any Mac. Recent versions of macOS list many developer files under their own Developer category, but not everything lands there.
• An app that misbehaves. Now and then an app writes far more logs or temporary files than it should. If System Data keeps growing fast, this is worth suspecting.

How do you clear System Data safely?

Work through these in order and check the Storage pane between steps.

1. Restart the Mac. This clears swap files and some temporary files in one go. On a Mac that hasn't restarted in a while it often frees more than any other step here, and it is completely safe. If you rarely restart, see how often you should restart your Mac.
2. Let Time Machine finish a backup. Connect your backup drive and let a backup complete. To see local snapshots, run tmutil listlocalsnapshots / in Terminal; it only lists them. macOS removes them when space runs low, so there is rarely a reason to delete them by hand. If you have no backup at all, set up Time Machine before deleting anything else.
3. Use the Developer category if you write software. In the Storage pane, open Developer and remove build caches and SDKs you no longer use.
4. Delete virtual machines you don't use, from inside the app that runs them.
5. Clear one large cache, if you find one. If ~/Library/Caches shows a single app holding many gigabytes, quit that app, then move the contents of its cache folder to the Trash. The app rebuilds what it needs, so the gain may be temporary, but it can help right before an update that needs room.
6. Empty the Trash. Files you deleted still take space until you do.

What to leave alone

Most of the damage people do to their Macs while chasing System Data comes from deleting things that looked like clutter.

• The System folder and anything under /private. These belong to macOS. Recent versions protect most of the system anyway, but /private/var holds things the Mac needs while it runs.
• Swap files and the sleep image. Deleting them while the Mac is running can cause crashes. A restart takes care of swap.
• Application Support and Containers. These hold app settings, libraries and sometimes your actual work. Remove an app's data through the app, or by uninstalling the app.
• Terminal commands from forums that use sudo rm on system paths. One typo can delete far more than intended.
• Cleanup tools that promise to free huge amounts of space. Much of what they sweep out is caches that rebuild within days, and a few go further than they should.

If the free space you get back after a restart and a backup is enough, stop there. Going after the last few gigabytes of System Data is rarely worth the risk.

How much System Data is normal?

There is no single right number. It depends on how much memory your Mac has, how long it has been since a restart, whether you use Time Machine, and what apps you run. On a Mac in daily use it is common to see it measured in tens of gigabytes, and more on Macs used for software development or virtual machines.

The figure that matters is free space. If the Mac has room to work, a big System Data number is mostly cosmetic. If the disk is close to full, start with the categories you can actually inspect, which usually free more space than System Data ever will, and what to clear when the startup disk is almost full walks through them.

A heads-up before space runs out

The best time to look at storage is before macOS warns you. Among the reminders Notchpals can bring down from your Mac's notch is one about storage, handled by its Tech care characters. If Gumdrop, a gumball robot, is the one you chose, it rolls out of the notch when free space drops below 10 percent, at most once a week.

GumdropYour disk is almost full. Let's sort the keepers from the stale ones!

Notchpals reads only how much space is free, never which files use it, and it never deletes anything, so what goes is up to you: the categories you can inspect first, then a restart, then any cache you recognize. Press Done when you have made some room.