What System Data is on a Mac, and how to shrink it
· 6 min read
System Data is not one thing. It is the label macOS puts on everything in the storage bar that is not a document, a photo, an app, music or mail: caches, logs, swap files, Time Machine local snapshots, application support folders, downloaded installers and the working files apps leave behind.
Shrinking it means working out which of those it actually is, because the fix for each one is different and only some of them are safe to touch. The number itself tells you nothing.
Where the number comes from
On Ventura and later, go to System Settings, General, Storage. On Monterey it is the Apple menu, About This Mac, Storage, Manage.
The bar at the top is a rough picture, not an audit. Hold the pointer over each segment to see its size. Below it is a list of categories, and a few of them (Applications, Documents, Music, iOS Files, Trash) open into a real list of files you can sort by size and delete from.
System Data does not. There is no detail view behind it, which is exactly why people end up searching for what it is. macOS is telling you a total without telling you what is in it.
Two things about that number are worth knowing before you start deleting. It is calculated in the background and it lags, sometimes by hours, so it will not drop the moment you empty something. And it includes space macOS considers reclaimable on demand, which is a separate confusion covered in purgeable space on a Mac.
The four things it usually turns out to be
In rough order of how often they are the answer:
Time Machine local snapshots. macOS takes hourly snapshots of your data volume and keeps them on the internal drive, so you can restore a file when the backup disk is not connected. On a machine that has not seen its backup disk in a month, these can be tens of gigabytes. Check with tmutil listlocalsnapshots / in Terminal. Time Machine local snapshots covers what to do about them.
Application support data and containers. Not the apps, the data underneath them: mail databases, message attachments, sync caches, virtual machine disk images, container images, simulator runtimes. These live in ~/Library/Application Support, ~/Library/Containers and ~/Library/Group Containers, and a single one of them can be larger than every document you own.
Caches and logs. ~/Library/Caches, /Library/Caches and /private/var/log. Individually modest, collectively not, and some apps never clean up after themselves.
Swap and the sleep image. macOS writes memory to disk when it runs short, and on a laptop it also writes a sleep image the size of your RAM. On current macOS these sit on their own hidden volume. You do not delete them by hand.
Measure it before you touch it
Finder will not show you this, because the folders involved are hidden. Two ways in.
In Finder, hold Option and open the Go menu; Library appears. Or press Command-Shift-Period in any Finder window to show hidden items until you press it again.
From Terminal, top down:
du -sh ~/Library/*to see which part of your own Library is heavy. Some lines will report permission errors; ignore them.sudo du -xhd 1 /System/Volumes/Data | sort -hto list the top level of the data volume by size, largest last. The-xkeeps it from wandering onto other volumes.du -shx /Users/*if more than one person uses the Mac. You will only get real numbers for accounts you can read, which is usually just yours.
One caveat on du: APFS lets two files share the same blocks on disk, and du counts each of them at full size. Totals can therefore come out higher than the space actually used. Treat the ranking as reliable and the absolute numbers as approximate.
If you would rather not do that by hand, Crumb is a menu bar app whose audit does exactly this pass across the whole Mac and breaks System Data into its parts: caches, logs, swap, temporary files and leftovers, plus local snapshots and iOS backups. The audit is read-only and free, so you can use it purely to find out where the space went and then decide what to do yourself.
What is safe to clear, in order
Work down this list and stop when you have enough space back. Do the reversible things first.
- Empty the Trash, including any Trash on external drives and any other account’s Trash you have access to.
- Delete downloaded installers. A
Install macOS <name>app in your Applications folder is usually 12 to 15 GB and does nothing after the update is done. - Delete old iPhone and iPad backups from
~/Library/Application Support/MobileSync/Backup, or from the Finder sidebar by selecting the device, Manage Backups. - Thin local snapshots, if that is where the space is.
- Clear caches for apps you have quit, one folder at a time from
~/Library/Caches, not the whole folder in one go. They regenerate; the cost is a slower first launch. - Remove build artifacts and package caches if you write software. Developer folders are the single most common cause of a System Data figure in the hundreds of gigabytes.
Restart afterwards. A lot of the space you freed will not be reported as free until logs, caches and snapshot accounting catch up.
What to leave alone
/System and /Library/Apple. The system volume is sealed and read-only on current macOS, so you mostly cannot damage it, but the parts you can reach are not worth the risk.
Swap files and the sleep image on the hidden VM volume. macOS manages them; deleting them by hand fixes nothing and a machine that is swapping heavily has a memory problem, not a disk problem.
Anything in ~/Library/Containers belonging to an app you still use. That is not cache, that is the app’s data: mail, notes, message history, licenses.
And be suspicious of any advice that tells you to delete a folder without saying what regenerates and what does not. If the numbers still do not make sense after all this, the Mac says storage is full but the folders do not add up covers the places Finder is not counting.
Questions
Why did System Data grow the moment I freed up space? Because the category is a remainder. When macOS reclassifies files, or a snapshot expires, space moves between categories before the total settles. Give it a restart and an hour before drawing conclusions.
Can I just delete everything in ~/Library/Caches? You can, and most of it comes back within a day, but do it with the affected apps quit and expect some of them to ask you to sign in again. Clearing one folder at a time is slower and much easier to undo mentally when something behaves oddly afterwards.
Is a cleanup app necessary for this? No. Everything above is Terminal and Finder. A cleanup app is worth it when you want the whole-Mac picture in one place, including the parts Finder hides, rather than assembling it from six commands.
My System Data is 200 GB and I do not write software.
Look at virtual machines, container images, and any app that syncs a large library locally. One of those is almost certainly it, and du -sh ~/Library/Application Support/* will name it in about ten seconds.