What Is System Data on a Mac? How to See What's Using Space
System Data is macOS's catch-all category for everything that Storage settings can't sort into Applications, Documents, Photos, Messages, iCloud Drive or macOS itself. It usually contains Time Machine local snapshots, caches, logs, virtual memory files, app support data, virtual machines and developer tools. iPhone and iPad backups can also end up here, depending on how macOS counts them.
A few dozen gigabytes of System Data is normal, and much of it shrinks or disappears on its own when your disk gets full. It is not a sign of a virus or a broken Mac. You don't need a "cleaner" app to fix it. You need to see what is actually there, then remove only what you no longer want.
Where macOS shows System Data
Open Apple menu › System Settings › General › Storage. The coloured bar at the top takes a moment to calculate, then shows each category. Older versions of macOS called the same category Other.
You can open most categories with the ⓘ button to see their files. System Data doesn't break down like that. It's a remainder: whatever is left once the other categories have been counted. That is why the number feels mysterious, and why so many articles guess at it.
What's actually inside System Data
Time Machine local snapshots
If you use Time Machine, macOS also keeps local snapshots on your startup disk, so you can restore recent versions even when your backup disk isn't connected. They are kept for a limited time and deleted automatically when space is needed. Right after a big change, such as deleting a large folder, the space may not come back straight away because a snapshot still references those files.
To list them, open Terminal and type:
tmutil listlocalsnapshots /
The simplest fix is usually to let Time Machine complete a backup and wait. If you really need the space now, you can delete one snapshot with sudo tmutil deletelocalsnapshots followed by its date (the part that looks like 2025-01-15-093012).
Caches
Apps store temporary copies of data to work faster: browser caches, thumbnails, downloaded updates, streaming buffers. They live mainly in ~/Library/Caches and /Library/Caches. Caches are meant to be disposable, but apps rebuild them as soon as you use them again. Deleting them gives you space for a while, not for good.
Virtual memory and the sleep image
When memory is full, macOS writes parts of it to swap files. On laptops, it also keeps a sleep image for hibernation. Both live in /private/var/vm and are fully managed by macOS. Don't delete them by hand. A restart often reduces swap usage.
iPhone and iPad backups
Backing up an iPhone or iPad to your Mac can take tens of gigabytes per device. Old backups of phones you no longer own are a classic source of lost space. They are stored in ~/Library/Application Support/MobileSync/Backup. Depending on your macOS version, they may appear as their own line in Storage (iOS Files) or be counted elsewhere.
To remove one properly, connect the device, select it in the Finder sidebar and click Manage Backups…, or use the ⓘ button next to iOS Files in Storage settings if it is shown.
App data, virtual machines and developer tools
Many large items live in ~/Library/Application Support and similar folders, which Storage settings often lumps into System Data. Common examples:
- Virtual machines and container disk images (for example Docker), which can be very large single files.
- Xcode data in
~/Library/Developer: simulators, device support files, build folders. - Music apps' sound libraries, game data, offline maps, downloaded AI models.
- Mail attachments and message databases.
Logs and diagnostic reports
System and app logs (/private/var/log, ~/Library/Logs) are usually small. An app that crashes in a loop or writes a log non-stop can still make them grow.
"Purgeable" space: why the numbers don't match
macOS separates space that is truly used from purgeable space: data it can remove by itself when you need room. That includes some caches, local snapshots and, if Optimise Mac Storage is on for iCloud Drive, local copies of files that are safely stored in iCloud.
This is why different places show different numbers. Select Macintosh HD in Finder and choose File › Get Info: the available space shown there may include purgeable space. Storage settings, Disk Utility and third-party tools may count it differently. None of them are wrong. They answer slightly different questions.
The practical takeaway: if macOS says space is purgeable, you rarely need to do anything. It will be freed when an app or a download actually needs it.
How to see what's really taking up space
1. Start with Apple's recommendations
In System Settings › General › Storage, macOS suggests storing files in iCloud, optimising storage, emptying the Trash automatically and reviewing large files. Use the ⓘ buttons next to Applications, Documents and the other categories to sort items by size. This often finds the big wins before you even look at System Data.
2. Look inside your Library folder
Finder hides your Library folder. To open it, choose Go in the menu bar and hold Option: Library appears in the menu. Or press Shift-Command-G and type ~/Library.
The folders worth checking first are Application Support, Caches, Containers, Developer and Group Containers. Finder doesn't show folder sizes in list view by default. Choose View › Show View Options and tick Calculate all sizes.
3. Measure folders in Terminal
For a quick ranking of what's heavy in your Library, run:
du -sh ~/Library/* 2>/dev/null | sort -h
The largest folders come last. You can run the same command on any subfolder to dig further. Some protected folders can't be read until you give Terminal Full Disk Access.
4. See it at a glance with Space Lens
Measuring folder by folder works, but it's slow when the space is spread across dozens of places. Space Lens in Folia Files draws a folder or a whole disk as bubbles sized by their weight, with the same items listed by size next to them. Choose View › Disk Usage (⌘6) in any pane, then click a bubble to go inside it. It works in folders Finder hides, such as your Library.
Its bar at the bottom of the list splits the disk into used, purgeable and free space, so you can see at once how much macOS would free on its own. Cloud folders (iCloud Drive and the other File Provider clouds) aren't scanned, so files that only live online don't inflate the numbers.
In the Downloads folder, for example, it takes a second to spot two copies of the same Xcode archive and an old disk image:
A few practical points:
- Tick the items you want to remove, then click Review and Delete. They go to the Trash, and you can undo with ⌘Z. Every operation is also kept in the history of the Operations button in the toolbar.
- To read protected folders (Mail, Safari, Messages…), give Folia Files Full Disk Access in System Settings › Privacy & Security › Full Disk Access.
- Space Lens and cleanup are available in both the website edition and the Mac App Store edition. In the App Store edition, Folia Files can only open what you have allowed in an Open panel, because of Apple's sandbox. Allow Macintosh HD once (Folia Files asks when needed, or go to Settings › Permissions). Full Disk Access doesn't lift that sandbox.
Folia Files also finds duplicates and has an app uninstaller, which help with the space counted outside System Data.
What is safe to remove, and what isn't
Usually worth removing, once you've checked what it is:
- Old iPhone and iPad backups, through Manage Backups… in Finder.
- Virtual machines, container images and simulators you no longer use, from the app that created them when possible.
- Data left by apps you have uninstalled, in
~/Library/Application Supportand~/Library/Containers. - Large downloads, installers and disk images (
.dmg) sitting in Downloads.
Fine to clear, but it comes back: caches. Clearing them can fix a misbehaving app, but it won't keep space free for long.
Leave alone:
- Anything in
/System, and swap files in/private/var/vm. - Files you don't recognise in
/privateor/Library. If you can't tell what something is, search its name before deleting it. - Local Time Machine snapshots, unless you really need the space immediately. macOS removes them on its own.
Be wary of tools that show a huge red "junk" number and offer to delete it in one click. Much of that "junk" is purgeable data macOS would have freed anyway, or caches that will be rebuilt within days.
When System Data keeps growing
If System Data grows steadily week after week:
- Restart your Mac. It clears temporary files and reduces swap.
- Let Time Machine finish a backup to an external disk, so older local snapshots can go.
- Measure again with the Terminal command above or with Space Lens, and compare with an earlier measurement. The folder that grew is your culprit.
- Check the app it belongs to. A sync tool, a virtual machine, a mail database or a runaway log is far more common than a system problem.
Once you can see where the space actually goes, System Data stops being a mystery. It becomes a short list of folders, most of them belonging to apps you know.
