What Is “System Data” on Mac?
You open Storage Settings, and something called System Data is eating 60 GB. Apple tells you nothing about what's in it. Here is the actual breakdown, and which parts you can safely get back.
“System Data” (called “Other” before macOS Monterey) isn't a real category. It is the leftover bucket: every file macOS can't confidently label as an app, a document, a photo, or a message gets counted there. That is why it swells to 30–80 GB on a machine a few years old, and why Apple's own Storage pane gives you no way to open it.
It is a mix of things. Some of it you should never touch. A meaningful slice of it is disposable.
What is inside System Data
1. Caches, usually the biggest reclaimable chunk
Every app writes to ~/Library/Caches, and the system writes to /Library/Caches. Browsers are the worst offenders: Chrome, Safari and Firefox each routinely hold 1–4 GB. Design and media apps cache preview renders that can run to tens of gigabytes.
Caches exist to make things faster on second use. Deleting them costs you a slower first launch and nothing else, the app regenerates what it needs.
2. Time Machine local snapshots
This is the one that surprises people. Even with no external drive attached, Time Machine takes hourly local APFS snapshots and keeps them on your internal disk. macOS thins them automatically when space runs low, but until it does, they can occupy many gigabytes and they land squarely in System Data.
You can see them yourself:
tmutil listlocalsnapshots /3. Swap and sleep image
When RAM fills, macOS pages memory to disk in /private/var/vm. The sleep image alone is roughly the size of your installed RAM. This is working memory, leave it alone. It shrinks on its own after a restart.
4. Logs and diagnostic reports
Crash reports, panic logs, and analytics in ~/Library/Logs and /private/var/log. Individually tiny, collectively a few gigabytes on an old install. Safe to clear.
5. Developer data, enormous if it applies to you
If Xcode is installed, System Data is likely dominated by it: DerivedData, iOS DeviceSupport, and simulator runtimes routinely total 20–50 GB. We cover this in detail in safely deleting Xcode DerivedData.
6. Application support files that outlived their app
Dragging an app to the Trash leaves ~/Library/Application Support, preference files, and container directories behind. Years of uninstalled apps accumulate here, and macOS counts every byte as System Data.
How to shrink it, in order of payoff
- Restart. Genuinely. This clears swap, the sleep image, and a good deal of temporary state. It is the single highest-ratio action and it takes two minutes.
- Empty the Trash: including in Photos and Mail, which keep their own.
- Clear browser and app caches. The largest safe win on most machines.
- Thin local snapshots. macOS does this on its own under pressure, but you can force it.
- Remove leftovers from apps you already deleted. Tedious by hand. This is where the folder-by-folder approach breaks down.
- Clear developer caches if you write software.
What not to delete
Some advice online tells people to delete the whole of ~/Library, or to remove files from /System or /private/var/db. Do not. You will break Mail indexes, Spotlight, licence records, and in the worst case the ability to boot. System Integrity Protection will stop the most catastrophic attempts, but it doesn't protect your user Library.
The rule that keeps you safe: delete caches and derived data, which regenerate. Do not delete databases and state, which don't.
See exactly what your System Data is made of
CleanMachine breaks the bucket down by category, caches, snapshots, logs, developer junk, orphaned app data, with real sizes. Scanning is free and takes about 60 seconds.
↓ Download Free & ScanWhy the number never quite matches
Storage Settings is an estimate, and it updates lazily. After a big cleanup it can take a reboot and several minutes of indexing before the figure moves. If you cleared 20 GB and the bar hasn't budged, wait rather than deleting more.