What “System Data” is on your Mac, and what’s safe to delete

System Data is the catch-all category in System Settings > General > Storage. Apple describes it as files that don’t fit any other category, mostly things macOS uses to run: caches, logs, VM files, temporary files, fonts, app support files and plug-ins. You can’t clear it from Storage settings, and you shouldn’t try by deleting system folders. What you can do is find out which parts of it are big on your Mac, then shrink those with the tools that made them. This guide shows you how, using only what’s built into macOS.

What Apple counts as System Data

Apple’s Storage settings guide puts System Data in a group with macOS and Other Users & Shared: categories that only show how much space they use. There’s no info button and no Delete. In Apple’s words, “You can’t manage the contents of this category.” The guide adds that macOS manages the contents, and that the size varies with the current state of your Mac. The wording is the same in the guides for macOS Sequoia 15, macOS Tahoe 26 and macOS 27, the current version as this is written.

It helps just as much to know what isn’t System Data. Storage settings gives these their own categories:

• iOS Files holds iPhone and iPad backups and firmware.
• Applications holds the apps you installed, including any “Install macOS” app you downloaded.
• Documents holds the other files in your home folder and has a Downloads button, so leftover disk images and installer packages in your home folder show up there.
• Mail, Messages, Photos and Trash each have their own bar.

So if you’ve been told to delete old iPhone backups or installers to shrink System Data, look at those categories instead.

You may also see purgeable space in Disk Utility. Apple describes it as space macOS can free when needed by removing files, and counts it as available. You can’t remove purgeable files yourself; macOS removes them as space is required.

Why the bar shrinks on its own

Much of what macOS keeps for itself is temporary by design:

• Time Machine local snapshots. Time Machine saves a snapshot about every hour, keeps it for 24 hours, and deletes snapshots as they age or when space is needed (Apple).
• Caches. Apple’s developer documentation defines them as data that can be regenerated as needed. Starting up in safe mode clears certain system caches, and Apple suggests it when you need room only for a while, such as during a macOS update (Apple).
• Swap. When apps need more memory than your Mac has, macOS writes some of it to disk, and that use rises and falls with what you have open.

That’s why System Data can be smaller after a restart, or a day later, without you doing anything. Before you delete anything, restart, wait a day if you can, and look again.

Find out what’s in your System Data

Here’s a method that uses only Storage settings, Finder, Disk Utility and a few read-only Terminal commands. It starts with items that can be very large on their own, then works through the folders where the rest collects. By the end you’ll have a short list.

Step 1: Write down the starting numbers

Open System Settings > General > Storage and wait for the bar to finish. Note the System Data figure. Click the info button next to iOS Files, Applications and Documents to see what’s in them: anything there is already counted outside System Data. Then click All Volumes, which shows a bar for each volume on your internal disk.

Step 2: Check Time Machine local snapshots

Local snapshots exist only on APFS disks with automatic Time Machine backups turned on. To list them, open Terminal and run:

tmutil listlocalsnapshots /

Time Machine’s snapshots are named com.apple.TimeMachine followed by a date. To see their sizes, open Disk Utility, choose View > Show APFS Snapshots and select your startup volume. The table at the bottom lists each snapshot, and Show Columns in its menu adds Private Size and Cumulative Size.

Apple says your Mac counts snapshot space as available storage, so snapshots shouldn’t stop you downloading files or installing software. Apple doesn’t say whether Storage settings includes them in System Data, so use this step to rule snapshots in or out, not to match numbers exactly.

Step 3: Check container and virtual machine disks

Docker Desktop stores all its Linux containers and images in one large disk image file. Its own Settings > Resources > Advanced shows where that file is and how much space it actually uses; Docker warns that other tools may report its maximum size instead. From Terminal:

docker system df

Virtual machine apps need a virtual disk for each machine, and it lives on your Mac unless you put it elsewhere. Check each machine’s size in the app that runs it.

Step 4: Size your Library folder

The caches, logs and app support files Apple mentions live in Library folders, and the one you control is the Library in your home folder.

In Finder, choose Go > Go to Folder, type ~/Library and press Return. Switch to List view, choose View > Show View Options, select “Calculate all sizes”, then click the Size column header to sort. Folder sizes take a while to fill in.

Or in Terminal:

du -sh ~/Library/Caches ~/Library/Logs ~/Library/Application\ Support 2>/dev/null
du -hd 1 ~/Library 2>/dev/null | sort -h | tail -n 15

The first line totals three folders. The second lists the 15 biggest folders in your Library, largest last. Both only read. On a big Library they can take minutes. macOS privacy protections cover some folders, so you may get a prompt asking whether Terminal can access other apps’ data. You can say no: du skips those folders, and 2>/dev/null hides the “Operation not permitted” lines.

Skip Mail and Messages in the results, since Storage settings counts them separately. Also, du -h counts in units of 1,024, while macOS reports storage in decimal units (a gigabyte is a billion bytes), so du figures look about 7% smaller. Use du -s --si instead of du -sh if you want them to line up.

Step 5: Look inside Caches and Logs

Open the biggest folders from Step 4. Apple’s convention is for each app to keep its cache in a folder named after its bundle identifier, which looks like com.company.AppName, so you can usually tell whose cache is big. ~/Library/Logs holds log files apps write, which you can read in the Console app. The system’s own log store sits in /var/db/diagnostics (see man log), and macOS looks after it.

Step 6: Read, but don’t touch, swap and the sleep image

sysctl vm.swapusage
pmset -g | grep hibernatefile

The first shows how much swap is in use. The swap files live on a hidden VM volume (Apple Platform Security). The second shows where macOS writes its sleep image: by default, Mac laptops store a copy of memory on disk when they sleep (man pmset). Both are part of how macOS runs.

Your list

You now know which of these matter on your Mac:

What’s safe to remove, and how

Time Machine local snapshots

Usually you don’t need to do anything. To clear them now, Apple’s method is to pause automatic backups: go to System Settings > General > Time Machine, click Options, set Backup Frequency to Manually, click Done, wait a few minutes, then turn automatic backups back on. Time Machine remembers your backup disks.

From Terminal, tmutil can thin them:

tmutil thinlocalsnapshots / 20000000000 4

That asks macOS to try to reclaim 20 GB (the amount is in bytes) by thinning snapshots, at urgency level 4 on the manual’s scale of 1 to 4. If it reports a permissions error, run the same command with sudo in front; this one only asks Time Machine to thin its own snapshots.

Docker and virtual machines

Use the tool, not Finder. docker system prune removes stopped containers, unused networks, dangling images and build cache, and Docker says freeing the space can take a few minutes. Don’t move or delete the disk image file by hand: it holds every container and image. For virtual machines, delete the ones you no longer need from inside the app that runs them.

Caches and logs

Quit the app, move its folder in ~/Library/Caches to the Trash, then empty the Trash; space isn’t freed until you do (Apple). Expect the cache to grow back as you use the app. Do the same for an app’s old files in ~/Library/Logs.

Mail downloads

Mail keeps some attachments temporarily in a Mail Downloads folder in your Library. In Mail > Settings > General, “Remove unedited downloads” controls when they’re deleted; by default an unedited attachment goes when you delete its message (Apple).

App support data

Apple’s developer documentation says Application Support can hold files with user data, so don’t clear it wholesale. Remove a folder only when you’ve removed its app and know you don’t want what’s inside.

Device backups, installers and updates

These usually aren’t System Data, but they’re often what people mean. Delete old iPhone and iPad backups in Finder: connect the device, select it, click Manage Backups in the General tab, then right-click a backup and choose Delete (Apple). Remove installers from the Applications or Documents category in Storage settings. Leave macOS update files to Software Update; if there isn’t room to install, Apple suggests starting up in safe mode.

What not to do

• Don’t delete anything in /System. By default, no process can write to the system volume, not even Apple’s own, and on macOS 11 and later your Mac starts up from a snapshot of it (Apple Platform Security).
• Don’t delete things in /private/var by hand. It holds working data macOS manages, including the system log store and, on Macs that use one, the sleep image.
• Don’t use sudo rm on anything you don’t recognise. sudo runs the command as root, past the permission checks that normally stop you, and rm skips the Trash, so there’s no undo.
• Don’t delete other snapshots. In Disk Utility, only snapshots named com.apple.TimeMachine belong to Time Machine. Leave anything else alone.
• Don’t change sleep settings to save space. The pmset manual asks you to use caution with its hibernation settings.

If you’d rather see all of this on one map, Disk Whisk scans without changing anything, explains each item it finds, and moves only what you approve to the Trash.