Quick answer: System Data over 15–20 GB usually means accumulated caches, old Xcode files, Time Machine snapshots, or log files. A normal healthy Mac should have 8–15 GB of System Data. Anything beyond that is likely recoverable.
How to Clean System Data on Mac
System Data is the most confusing entry in macOS Storage. It can grow to 40–80 GB over time and macOS gives you no easy button to clean it. Here's what's inside and the specific steps to reduce it.
What's Actually Inside System Data
Time Machine local snapshots
5–30 GBmacOS keeps local snapshots before syncing to your backup drive. These are invisible in Finder. They get cleared when macOS needs space, but sometimes they linger.
App caches and logs
5–15 GBEvery app you use writes cache files and logs. Browser caches alone can be 2–5 GB. Xcode, Slack, and Creative Cloud apps tend to be the worst offenders.
Xcode derived data
10–30 GBIf you develop on your Mac, Xcode stores build artifacts here. Xcode rebuilds these automatically — they're always safe to delete.
iOS device backups
5–30 GBiTunes/Finder backups of your iPhone and iPad. Each full backup is 5–15 GB. You might have multiple old ones.
Mail attachments and database
2–10 GBMail stores all attachment data locally. If you've been using Mail for years, this grows considerably.
Virtual machine files
20–100 GBParallels, VMware, or UTM disk images are stored in System Data. These are the biggest single items you might find.
How to Clean System Data: Step by Step
1Delete Time Machine local snapshots
Open Terminal and run this command to list snapshots:
To delete all local snapshots:
This is safe — your files are not deleted, only the local Time Machine snapshots.
2Delete Xcode derived data (developers only)
In Finder, press ⌘ + Shift + G and navigate to:
Select all contents and delete. Xcode will rebuild this next time you compile. Also check ~/Library/Developer/Xcode/iOS DeviceSupport — delete old iOS version folders you no longer need.
3Delete old iPhone backups
Open Finder → click your iPhone in the sidebar (if connected) → Manage Backups. Alternatively: System Settings → General → Storage → iOS Files. Delete backups older than your most recent one.
4Clear application caches
In Finder, navigate to ~/Library/Caches and look for the largest folders. Browser caches, Slack, and creative apps tend to be biggest. Delete folders you recognize — they regenerate automatically.
~/Library/Caches, not files in ~/Library/Application Support — that's where apps store user data.5Check for virtual machine disk images
Parallels and VMware store their disk images in ~/Parallels or ~/Documents. These can be 30–80 GB each. If you no longer use the VM, delete the disk image from within the app before uninstalling.