Why is System Data so big on a Mac, and how do you shrink it?

System Data is the grey part of the bar in System Settings › General › Storage, and the only category there you can't click into. It is a catch-all: whatever doesn't fit Applications, Documents, Photos or the other categories ends up in it. Tens of gigabytes is normal. When it reaches a hundred or more, one or two specific things have usually grown, and you can find them.

Published October 21, 2026 · Tested on macOS 27 · 9 min read

The short version

  • System Data is mostly local Time Machine snapshots, caches, app data, virtual machine images, logs and leftovers from apps you deleted.
  • Check snapshots first: tmutil listlocalsnapshots /. They are often the biggest part, and removing them is safe.
  • Then sort your Library folders by size. The biggest item is often not a cache but an app's data folder.
  • Never delete whole Library folders or anything in /System. Delete the specific items you have identified.

What System Data actually is

Apple describes System Data as files that don't belong to any other category. That mainly means files the system itself uses, such as logs, caches, virtual memory files and other runtime resources, plus temporary files, fonts, app support files and plug-ins. macOS manages these files, which is why Storage settings gives you no button to manage them.

In practice, the large items in System Data almost always come from this list:

What it looked like on a real Mac

On the Mac this guide was written on, ~/Library/Application Support was 18 GB, more than four times the 4.3 GB in ~/Library/Caches. A single root-only system cache, the one macOS uses to symbolicate crash reports (com.apple.coresymbolicationd in /System/Library/Caches), held about 9 GB. Storage settings shows neither of them anywhere except inside System Data.

Step 1: Rule out local snapshots

tmutil listlocalsnapshots /

If that prints a long list after you recently deleted a lot of files, the snapshots are probably holding that space. Removing them with sudo tmutil deletelocalsnapshots / is safe: it doesn't touch the backups on your Time Machine disk. The purgeable space guide explains what you give up.

Step 2: Find the big folders

Terminal can only see inside every folder if it has Full Disk Access. Turn it on in System Settings › Privacy & Security › Full Disk Access, and turn it off again when you are done. Then list the largest folders, one level at a time:

du -hd 1 ~/Library 2>/dev/null | sort -h | tail -12
du -hd 1 ~/Library/"Application Support" 2>/dev/null | sort -h | tail -12
du -hd 1 ~/Library/Containers 2>/dev/null | sort -h | tail -12

If you prefer Finder, choose Go › Go to Folder, type ~/Library, switch to list view and turn on View › Show View Options › Calculate all sizes. Folders in Containers are named after the app's bundle identifier, for example com.docker.docker, so search the name if you don't recognise it.

Step 3: Clear what is safe to clear

Caches

An app rebuilds its cache when it needs it, so deleting a cache folder is safe. The cost is a slower first launch while the app fills it again. Quit the app first, then delete the folder inside ~/Library/Caches that belongs to it. Don't delete the Caches folder itself.

System caches in /Library/Caches and /System/Library/Caches need an administrator, and macOS protects some of them. The simplest safe way to reset them is to start up in safe mode once. On a Mac with Apple silicon, shut down, hold the power button until the startup options appear, select your disk, hold Shift and choose Continue in Safe Mode. On an Intel Mac, hold Shift while it starts up. macOS clears some system caches in safe mode and rebuilds them afterwards.

Logs and crash reports

Old logs in ~/Library/Logs and crash reports in ~/Library/Logs/DiagnosticReports and /Library/Logs/DiagnosticReports are safe to delete, unless you are troubleshooting a problem and someone has asked you for them.

Leftovers from apps you removed

When you delete an app, its data usually stays behind in several places:

Delete these only for apps that are really gone. A folder that belongs to an app you still use is that app's data: its settings, its database, sometimes your documents.

iPhone and iPad backups

Each local device backup in ~/Library/Application Support/MobileSync/Backup can be tens of gigabytes, and backups of devices you no longer own stay forever. Delete them in Finder: connect any iPhone or iPad, select it in the sidebar, click Manage Backups in the General tab, then select the backups you don't need, including those of old devices, and click Delete Backup.

Virtual machines and Docker

Delete virtual machines from inside the app that created them (Parallels Desktop, UTM, VMware Fusion) so it doesn't keep a broken entry. For Docker images, build cache and volumes, follow the Docker section of the developer storage guide.

What not to delete

Why System Data grows back

Some of it always will. Time Machine takes a new snapshot every hour, apps refill their caches, logs keep being written and Spotlight updates its index. System Data going up and down by a few gigabytes a day is normal. Storage settings also calculates its numbers in the background, so it can take a while to show space you have just freed.

What should not happen is steady growth by tens of gigabytes. That usually points to one app: a sync client re-downloading, a VM growing, a log file that never rotates. Steps 2 and 3 above will find it.

Doing it with CleanMacster

CleanMacster takes System Data apart in the same order, with every item listed with its path and size before anything is removed:

Items that may hold personal data are not selected for you, and a single setting sends everything to the Trash instead of deleting it.