What is “System Data” on a Mac, and why is it so big?
Open System Settings → General → Storage and you will usually see a grey bar labelled System Data (older macOS versions called it “Other”). Apple does not break it down, which is why it often looks like a mystery.
What counts as System Data
It is everything macOS cannot file under Apps, Documents, Photos, Mail and the other named categories. In practice that is a mix of:
- App and system caches in
~/Library/Cachesand/Library/Caches - Time Machine local snapshots
- iPhone and iPad backups
- Xcode Derived Data and old iOS simulator runtimes
- Docker and virtual-machine disk images
- Logs, crash reports and diagnostic data
- Parts of macOS itself that are not counted as System
Why it grows
Most of these are created automatically and rarely cleaned up. Caches rebuild, snapshots wait for the next backup, and developer tools keep every build and simulator you ever installed. Some of it, like snapshots and purgeable caches, macOS frees by itself when it needs the room, which is why the number can shrink and grow on its own.
What to do about it
Don’t delete folders blindly. First find out which categories are large on your Mac, then deal with those one at a time. These guides cover the common ones: Time Machine snapshots, iPhone backups and Xcode data.
A normal size is tens of GB. If yours is over 100 GB, one or two of the items above is almost always responsible.
McCleaner scans for free and changes nothing until you pick items to clean.
Get McCleaner
Disclosure: this guide is published by the developer of McCleaner. Scanning is free; cleaning is paid, from €1.99. Back up important data before deleting anything. Commands are provided as is.