What Is System Data on Mac and How to Reduce It

System Data is the catch-all slice of Mac storage. Learn what it holds, why it swings in size, and the safe ways to shrink it, from snapshots to old backups.

MacWipe 31 October 2026 8 min read

System Data is the catch-all category in your Mac’s storage bar. Apple describes it as a general category that measures the space used by all Apple and third-party files that don’t belong to a more specific category. You can shrink it safely by restarting, deleting old iPhone backups, clearing caches and developer leftovers, and thinning local Time Machine snapshots.

The number is often large because so many things land in it. This guide explains what is inside, why it changes from day to day, and which parts you can touch without risking your Mac.

Where to see System Data

Open the Apple menu and choose System Settings.
Click General in the sidebar, then Storage.
Wait a few seconds for the bar to finish calculating. System Data is the grey section, listed at the bottom of the category list.

This path is the same on macOS Tahoe 26, Sequoia 15 and Sonoma 14. Apple’s list of categories is not the same as a list of folders, so there is no single “System Data” folder to open. Unlike Documents or Applications, System Data has no More Info button that lets you browse and delete items.

What counts as System Data

Apple does not publish a full breakdown. In practice these are the usual contents:

Caches. Temporary files made by macOS, apps and browsers. They rebuild when needed.
Logs. Diagnostic records written by the system and by apps.
Local Time Machine snapshots. Hourly copies of your startup disk, kept on the Mac itself.
Virtual memory and sleep files. Swap space and the file macOS writes so it can restore your session after sleep. These are managed by macOS and resize on their own.
App support data. Plug-ins, extensions, fonts, databases and other files apps keep outside their own bundle.
iPhone and iPad backups. Depending on how your Mac classifies them, these can sit in System Data instead of a named category.
Developer data. Xcode build files, simulators and similar tools.

Why it grows and shrinks

System Data is not fixed. Your Mac fills it as you work and trims it when space gets tight.

Apple states that Time Machine keeps a local snapshot for 24 hours, keeps one more snapshot of your last successful backup until space is needed, and deletes snapshots as they age or as space is needed. A macOS update also saves a snapshot first. That is why System Data can jump by tens of gigabytes around an update, then fall again a day later.

The Storage screen also takes a moment to recalculate. A figure that looks stuck after a clean-up may simply be out of date. Restart and check again before deciding nothing worked.

Back up first

Before you delete anything in bulk, make sure you have a recent Time Machine backup or another copy of your important files. Everything below is reversible or regenerable, except deleted iPhone backups and anything you remove from the Trash.

Safe ways to reduce System Data

1. Restart your Mac

A restart clears temporary files and lets macOS release swap space. It costs nothing and is the first thing to try. After the Mac starts, wait a few minutes, then reopen Storage.

2. Empty the Trash

Apple notes that a file you move to the Trash does not free space until you empty it. Control-click the Trash in the Dock and choose Empty Trash. Check what is in there first, because this step cannot be undone.

3. Delete old iPhone and iPad backups

Old device backups are one of the most common reasons for a huge System Data figure. Each can run to many gigabytes. Apple’s guide to locating and managing backups gives these steps:

Open Finder and connect your iPhone or iPad with a USB cable.
Select the device in the Finder sidebar. Tap Trust on the device if asked.
Click the General tab, then Manage Backups.
Right-click the backup you no longer need and choose Delete.
Click OK.

Apple warns that a deleted backup can’t be recovered. If you want to keep one, choose Archive instead, so later backups don’t replace it. Backups are stored in ~/Library/Application Support/MobileSync/Backup/. Use the Finder window above rather than deleting folders from there by hand.

4. Clear caches

User caches live in ~/Library/Caches. Browser caches, app caches and download leftovers add up here. The safe approach is to open an app’s folder inside Caches, remove its contents, and leave the folder itself in place. Quit the app first. Do not delete the whole Library folder. Our guide on how to clear cache on Mac walks through each location.

5. Thin local Time Machine snapshots

Apple has no button for deleting snapshots. Its documented method is to switch Time Machine’s automatic backups off for a few minutes, then back on:

Open System Settings > General > Time Machine.
Click Options.
Set Backup Frequency to Manually and click Done.
Wait a few minutes while Time Machine removes its local snapshots.
Reopen Options, set the frequency back to automatic, and click Done.

This is the route to try first. If you want to see and control the snapshots, Terminal has tmutil, Apple’s own Time Machine tool. Listing is harmless:

tmutil listlocalsnapshots /

Each line looks like com.apple.TimeMachine.2026-10-31-093000.local. The date part, 2026-10-31-093000, is what you pass to the delete command:

sudo tmutil deletelocalsnapshots 2026-10-31-093000

The built-in manual (man tmutil) describes this as deleting all local Time Machine snapshots for that date. Run it once per snapshot you want gone. There is also tmutil thinlocalsnapshots, which asks macOS to reclaim a set number of bytes with an urgency from 1 to 4, but it frees an unpredictable amount.

Only delete snapshots you have listed yourself, and only the Time Machine ones. Local snapshots do not replace a real backup, so if your external backup drive is not working, fix that before removing them.

6. Remove developer data

If you build apps, Xcode is often the largest single contributor.

DerivedData. In Xcode, open Xcode > Settings > Locations and click the small arrow beside Derived Data to reveal it in Finder, usually ~/Library/Developer/Xcode/DerivedData. Quit Xcode and delete the project folders inside. Xcode rebuilds them next time.
Old simulators. To remove simulators that no longer match your installed Xcode, run xcrun simctl delete unavailable in Terminal.

Package manager caches from tools such as npm, Homebrew or Gradle also pile up. Each tool has its own clean command, so check its documentation before deleting anything by hand.

7. Look at the big things outside System Data

Sometimes the real problem is not System Data at all. Large downloads, Mail attachments and old projects shrink the free space just the same. See how to find large files on Mac, and remove apps you no longer use with the steps in how to uninstall apps on Mac.

What not to touch

Anything in /System. macOS protects it for good reason.
Anything under /private/var. This includes the virtual memory and sleep files. macOS creates and resizes them itself.
The whole ~/Library folder. It holds app settings, mail, keychains and saved data.
Folders you don’t recognise inside Application Support. Deleting an app’s support folder can wipe its settings or its library.
Snapshots made by other software. tmutil is for Time Machine snapshots. Don’t remove ones that other tools created.
Your only iPhone backup. Once it is deleted, it is gone.

Unexplained gigabytes are not a reason to start deleting system files. If the number still looks wrong after a restart and the steps above, leave it alone.

A faster way: MacWipe

Hunting through folders is slow and easy to get wrong. MacWipe is a Mac cleaner for macOS 14 or later, on Apple silicon and Intel, and it is signed and notarised by Apple. Its Smart Scan checks caches, logs, the Trash, app leftovers and browser data in one pass. The Storage tool shows what is taking up space, and Developer finds build folders and caches from developer tools.

MacWipe lists every item and explains why it is safe before anything is removed. Nothing goes without your say. Scanning is free and so is your first clean. After that it costs £23.99 a year, with monthly, quarterly and £59.99 lifetime options, and one plan covers up to three Macs. See pricing. There is a 14-day money-back guarantee, and no tracking or analytics.

FAQ

Is it safe to delete System Data on Mac?

You can’t delete the whole category, and you shouldn’t try. It holds files macOS needs. Remove only the parts above that are safe: old backups, caches, snapshots and developer build files.

Why is my System Data so big?

The usual causes are local Time Machine snapshots, old iPhone backups, large caches and Xcode data. A recent macOS update can also inflate it for a day or two. Check each in turn, starting with backups.

Why did System Data not shrink after I cleaned up?

The Storage screen can lag. Restart your Mac, wait a few minutes, and open Storage again. Local snapshots also hold space until Time Machine releases them, so thin those if you haven’t.

Will System Data go down by itself?

Often it does. Time Machine removes snapshots as they age or when space is needed, and caches rebuild only as required. If your disk is nearly full, macOS clears what it can on its own.