In short
- System Data is a leftover figure: the used space macOS couldn't put in any other category.
- It is mostly app data, caches, logs, local snapshots and system files.
- Caches, logs and the Trash are safe to delete. App data and system files are not.
- You can't delete System Data as a whole. You remove specific items inside it.
Why macOS shows it as one block
Open System Settings → General → Storage and you'll see categories such as Applications, Documents and Photos, and then System Data. macOS works out the named categories first. System Data is what remains: used space that didn't fit anywhere else. Howard Oakley of The Eclectic Light Company describes it as a category of leftovers, not a real one (source). Apple calls it a general category, for the Apple and third-party files that fall outside the more specific ones (Free up storage space on Mac).
That is why the number can be 50 GB on one Mac and 200 GB on another, and why there is no button to open it.
What is usually inside
| Part | What it is | Where it lives |
|---|---|---|
| App data | Settings, databases and downloaded content apps keep for you | ~/Library/Application Support, ~/Library/Containers |
| App caches | Temporary files apps keep to work faster | ~/Library/Caches |
| Logs | Diagnostic records from apps and macOS | ~/Library/Logs |
| Local snapshots | Time Machine copies kept on the internal disk | Managed by macOS |
| System files | Shared components, swap space and temporary files | /Library, /private/var |
On a developer's Mac the largest parts are often tools: Xcode build files and simulators, Docker disk images, and package manager caches for npm, Yarn or pip. See what is safe to delete on a developer's Mac.
What is safe to delete, and what isn't
- Browser cachesChrome, Safari and others download them again as you browse.Safe to delete
- LogsOnly useful when reporting a problem.Safe to delete
- Files in the TrashAlready deleted; emptying gives the space back.Safe to delete
- Other app cachesMost rebuild, but some hold downloads, such as offline music in Spotify.Check first
- Data left by deleted appsUsually unwanted, but check the app really is gone.Check first
- App dataMessage history, app settings and saved work live here.Keep
- System filesmacOS manages these itself.Keep
Don't delete folders from ~/Library because they are large. Size says nothing about safety. Application Support holds things like your Messages history and app licenses.
How to clear System Data on Mac
- Restart your Mac. This clears temporary files and swap space.
- Empty the Trash. Files in the Trash still use space.
- Clear your browser's cache from the browser's own settings. How, and which other caches are safe.
- Check local snapshots. In Terminal,
tmutil listlocalsnapshots /lists them. macOS removes them on its own when space runs low, so only delete one by hand (tmutil deletelocalsnapshotsfollowed by its date) if you need the space now. More on local snapshots. - Remove data left by apps you've deleted. Look in
~/Library/Application Supportfor folders named after apps you no longer have.
How StorageJanitor shows it
StorageJanitor scans the whole disk and draws one bar. The part macOS calls System Data is bracketed and split into app data, caches, system files and logs. Below it, the biggest items are listed with a plain name and a verdict.
This figure is measured from the files themselves, so it won't match System Settings exactly. Developer tools, device backups and snapshots, which macOS also counts as System Data, are shown as their own categories.
- Photos & videos 52.3 GB
- Documents 28.6 GB
- Applications 21.4 GB
- App data 22.6 GB
- App caches 13.1 GB
- System files 9.4 GB
- Logs 1.3 GB
Questions people ask
What is System Data on a Mac?
System Data is the storage category macOS uses for everything it can't place in another category such as Applications, Documents or Photos. It is mostly app data, caches, logs, local snapshots and system files.
Is it safe to delete System Data?
Parts of it. Caches, logs and files in the Trash are safe to remove because apps recreate them. App data, such as message history and app settings, is not. You can't delete System Data as a whole; you remove specific items inside it.
Why is System Data so large?
Because it is a leftover figure, it grows with anything macOS doesn't classify: browser and app caches, chat app media, local Time Machine snapshots, and for developers, tools like Xcode and Docker.
Does restarting reduce System Data?
Sometimes. A restart clears temporary files and swap space, which can free a few gigabytes. It doesn't remove caches or app data.
How do I see what is inside System Data?
System Settings doesn't show it. You can inspect ~/Library in Finder folder by folder, or use an app such as StorageJanitor that splits System Data into named parts and lists the largest items.