System Data Is Huge on Mac: How to Find What Is Using 100 GB or More
Quick answer: To find what is inside a huge System Data number, check these folders in order: ~/Library/Caches, ~/Library/Application Support, ~/Library/Developer, ~/Library/Application Support/MobileSync/Backup (iPhone backups), and Time Machine local snapshots. One Terminal command lists the biggest folders in your Library so you do not have to guess.
macOS lumps caches, logs, app support files and developer data into one grey System Data bar and gives you no breakdown. This guide shows how to get the breakdown yourself. For the general overview see How to Clear System Data on a Mac (and Why It Gets So Big).
Find the biggest folders in your Library
Open Terminal and run:
du -sh ~/Library/* 2>/dev/null | sort -hr | head -20The command prints the 20 largest folders in your Library, biggest first. It only reads sizes; it deletes nothing. It can take a minute on a full disk.
What the usual suspects are
| Folder | What it holds | Notes |
|---|---|---|
| ~/Library/Caches | App and system caches | Rebuilt by apps as needed |
| ~/Library/Application Support | App data, some caches | Big for chat, design and video apps |
| ~/Library/Developer | Xcode DerivedData, simulators | Often tens of GB for developers |
| ~/Library/Application Support/MobileSync/Backup | iPhone and iPad backups | One folder per device |
| ~/Library/Containers | Sandboxed app data, Docker | Docker images can be very large |
| /private/var/vm | Virtual memory swap files | Managed by macOS, do not delete |
Which are safe to clear
Caches are designed to be disposable. See How to Clear Cache on a Mac (App, Browser and DNS Cache).
Xcode DerivedData and old simulators can be deleted and are rebuilt when you next build.
iPhone backups are safe to delete only if you have another backup of the phone. See How to Clear Messages Storage and Delete Old iPhone Backups on a Mac.
Docker and Homebrew data can be cleaned from their own tools. See How to Remove Docker, Homebrew and Xcode From a Mac Completely.
Application Support belongs to your apps. Delete from it only for apps you have removed.
Do not delete system folders outside your home Library.
Time Machine snapshots
Local snapshots also count as System Data. macOS removes them automatically when it needs space, but you can inspect and delete them.
Doing it with an app
MacCove Space breaks System Data into named categories, grades every item Safe, Review or Caution, and lets you clear the safe ones. Clean-ups go to the Trash, so you can undo.
FAQs
Why is System Data 100 GB?
Because it holds caches, logs, backups, snapshots and developer data together. One of them has usually grown.
Does restarting reduce System Data?
Sometimes a little, as temporary files are cleared. It will not fix a large backup or cache.
Can I delete the Library folder?
No. Delete only specific items inside it that you can identify.