System Data is not one folder, it is a category. That means the fix is never a single delete, it is a short list of specific paths. These are the seven that account for the bulk of it on most Macs, in the order we usually work through them.
Open Finder, press Command Shift G, and paste each path to see it. The tilde means your home folder.
1. ~/Library/Caches
Every app keeps temporary files here. Browsers, Slack, Spotify and Discord are the usual leaders, and none of them clean up reliably.
du -sh ~/Library/Caches/* 2>/dev/null | sort -hr | head -10Safe to delete: yes, everything inside. Delete the contents rather than the folder itself so permissions stay intact. Expect the affected apps to feel slightly slow on first launch while they rebuild.
Typical payout: 5GB to 40GB.
2. ~/Library/Application Support/MobileSync/Backup
Local iPhone and iPad backups. Each device can hold one backup per iOS version, and old phones you replaced two years ago may still have theirs.
du -sh ~/Library/Application\ Support/MobileSync/Backup/*Safe to delete: yes, if you have iCloud backup enabled or do not need local restore points. Check the dates first. A backup from a device you no longer own is pure waste.
Typical payout: 20GB to 100GB.
3. ~/Library/Developer/Xcode/DerivedData
Build output for every project you have ever compiled with Xcode. Rebuildable in full, with the only cost being a slower first build.
rm -rf ~/Library/Developer/Xcode/DerivedData/*Safe to delete: yes, with Xcode closed. Keep the Archives folder next to it unless you are sure you no longer need the dSYMs for shipped versions.
Typical payout: 10GB to 80GB.
4. ~/Library/Caches/com.apple.bird
The iCloud Drive cache. If you keep a lot of documents in iCloud with Desktop and Documents syncing enabled, this grows steadily.
du -sh ~/Library/Caches/com.apple.birdSafe to delete: yes. It triggers a re-sync, so do it when you are on a good connection and not mid-upload. Your files in iCloud are not affected.
Typical payout: 1GB to 20GB.
5. Leftover app support folders
When you delete an app by dragging it to the Trash, its data folder stays behind. Over a few years this adds up to a surprising amount of dead weight.
ls -lt ~/Library/"Application Support" | tail -30Safe to delete: only for apps you have removed and do not plan to reinstall. Check the folder name against your Applications folder first, and never clear this directory in bulk: it also holds mail databases, save games and licence files for apps you still use.
Typical payout: 1GB to 15GB.
6. ~/Library/Logs
Diagnostic and crash logs. Individual files are small, the directory is not.
du -sh ~/Library/LogsSafe to delete: yes for the contents. Keep anything from the last day or two if you are troubleshooting a specific crash.
Typical payout: 0.5GB to 10GB.
7. Leftover system installers
ls -lh /Library/Updates/Folders with numeric names can be remnants of a macOS update that has already installed. Deleting them via the Trash is safer than rm here, and only after your Mac has been running stably on the new version for several days.
Safe to delete: yes, if the update is complete and stable. If anything is behaving oddly, leave them: they may be needed for recovery.
Typical payout: 2GB to 12GB.
macoptimize lists each of these as a named category with its measured size, and the Recommended preset only ticks the ones that regenerate on demand. Everything removed is written to the History tab, so you can see which category gave you what, and undo the guesswork next month.
The two folders you should not clear
- ~/Library/Application Support as a whole. It contains device backups, mail data and app state. Pick individual folders instead.
- /private/var/vm. Swap is managed by macOS. If it is large, the machine needs more memory headroom, not a delete command.
The seven folders at a glance
| Folder | Typical size | What happens after deleting |
|---|---|---|
| ~/Library/Caches | 5GB to 40GB | Apps rebuild what they need, first launches feel slower |
| MobileSync/Backup | 5GB to 60GB | Nothing, unless you still restore from that backup |
| Xcode/DerivedData | 15GB to 80GB | One full rebuild |
| Caches/com.apple.bird | 1GB to 20GB | iCloud Drive re-downloads on demand |
| Stale app support folders | 1GB to 15GB | Nothing, the app is already uninstalled |
| ~/Library/Logs | 1GB to 10GB | New logs start immediately |
| Leftover system installers | 6GB to 15GB | Nothing, the update is already installed |
A safe way to clear caches
The blunt version, rm -rf ~/Library/Caches/*, works but it also deletes caches that make the next hour unpleasant, including the ones your browser and editor are using right now. Filtering by age is gentler and reclaims most of the space, because the old entries are usually the large ones:
find ~/Library/Caches -mindepth 1 -maxdepth 1 -type d -mtime +30 -exec du -sh {} + | sort -hr | head -20Review the list, then remove the same set with -exec rm -rf {} +. Caches touched in the last month stay in place, which means the apps you actually use every day keep their warm state.
Before clearing a whole cache root, quit the apps that own them. Safari, Chrome and Xcode all hold files open, and deleting underneath a running app can leave that app behaving oddly until it is restarted.
Old iPhone and iPad backups
Device backups are the single largest item in most System Data scans, and they are also the item people are least sure about. Check what you have:
ls -lh ~/Library/Application\ Support/MobileSync/BackupEach folder is one backup, named by a device identifier. If iCloud Backup is on for that device and has completed successfully, the local copy is redundant. If it is not, deleting the local backup removes your only full copy of that phone, including app data that no longer exists anywhere else. Decide that per device rather than in bulk.
Checking that the space came back
df -h /Measure before the cleanup and again two minutes after. APFS holds deleted blocks while local snapshots reference them, so space can take a few minutes or a reboot to appear. If the number has not moved after a reboot, something else is holding the data, and a snapshot list is the first place to look.
What deleting caches slows down
Worth knowing so the slowdown is not mistaken for damage. Without caches, apps rebuild indexes, browser pages fetch their resources again, and Xcode compiles everything instead of reusing objects. On a modern Mac that means a handful of slower launches rather than a persistent change, and the caches refill in the background over the following days.
FAQ
Will deleting caches slow my Mac down?
For a short while, yes. Apps rebuild what they cached, so the first launch of each one is slower and web pages load slower on the first visit. Nothing is permanently slower, and the caches refill over the next few days of normal use.
Can I clear these from Finder instead of Terminal?
Mostly. Press Shift Command Period to show hidden folders, then navigate to ~/Library and open Caches. Finder cannot show sizes per folder in a list view, so sorting by size requires Get Info on each item, which is why the Terminal commands are faster for anything large.
How often should this be done?
Two or three times a year is plenty for most people. Developers should clear caches and build output monthly, because Xcode, Docker and package managers generate far more than a normal workload does. The sign to act is free space dropping below 15 percent, or warnings during an update.
Should I delete an iPhone backup if I use iCloud?
Only if iCloud Backup is enabled for that device and shows a completed recent backup. Otherwise the local folder is your only complete copy, including app data that cannot be recovered from anywhere else. Check the device settings before removing anything.
What happens if I delete the wrong folder?
Most cache folders regenerate and cost you a slow first launch. The genuinely risky ones are Application Support folders for apps you still use, Mail data, Photos libraries and signing keys or certificates. If a folder name looks like an app you still have installed, leave it alone.