What is System Data on a Mac?
It is the largest category in macOS Storage settings that Apple will not let you look inside. Here is what is actually in there, and how to reduce it safely.
System Data on a Mac is the storage category macOS uses for every file it cannot classify as Apps, Photos, Documents, Music or Mail. It is not one thing and it is not a bug.
In practice it is mostly cache files that apps write and never clean up, log files, old iOS device backups, disk images sitting in Downloads, virtual machine images, and support folders belonging to apps you deleted long ago. Time Machine's local snapshots are counted here too.
It grows without limit because nothing in macOS is responsible for shrinking it. Apple's Storage settings will show you how large System Data is but will not show you what is inside or let you remove any of it selectively. A System Data figure between 20 GB and 100 GB is completely normal; on one Mac we measured it held 84.8 GB of removable files, about 8.5% of a 994 GB disk.
Where to see System Data on your Mac
- Open System Settings → General → Storage.
- Wait for the bar to finish calculating — it can take a minute or two.
- Hover the grey segment at the right of the bar. It is labelled System Data.
Older versions of macOS called this Other storage. It is the same category under a different name.
What is actually inside System Data
Measured on a real 994 GB Mac running macOS 26, in order of size:
| What | Size | Where it lives |
|---|---|---|
| Dependency folders | 27.7 GB | Project folders, for anyone who writes code |
| App caches | 22.5 GB | Inside app profiles in Application Support |
| Package manager caches | 18.7 GB | Hidden folders in your home directory |
| System caches | 6.0 GB | ~/Library/Caches |
| Simulator data | 3.4 GB | ~/Library/Developer |
| Duplicate files | 2.05 GB | Scattered across Documents and Downloads |
| Old installers | 1.86 GB | ~/Downloads |
| Logs | 443 MB | ~/Library/Logs |
Your breakdown will differ. The pattern that holds for almost everyone is that a handful of folders account for the overwhelming majority, and none of them are visible in Storage settings.
How to reduce System Data on a Mac
In rough order of how much they return for the effort:
- Empty the Trash. Deleted files still count until you do.
- Delete old iOS backups. Go to
~/Library/Application Support/MobileSync/Backup. A single iPhone backup can exceed 100 GB, and iCloud Backup usually makes it redundant. - Clear your caches. Usually the largest single category after backups.
- Clear old installers. Sort Downloads by size and remove
.dmgand.pkgfiles for software you have already installed. - Remove leftovers from deleted apps. These never expire on their own.
- Thin Time Machine local snapshots with
tmutil thinlocalsnapshots / 21474836480 4in Terminal. This needs an administrator password.
MacBook Air and MacBook Pro storage full because of System Data
If your MacBook storage is full and Finder shows no files large enough to explain it, System Data is almost always the answer. The category is invisible in Finder, so the space appears to be consumed by nothing at all.
A MacBook Air with a 256 GB SSD is the machine this hurts most. The storage is soldered and cannot be upgraded, so 60 GB of System Data is close to a quarter of everything you own. The same 60 GB on a 1 TB Mac Studio is unremarkable.
The order to work through is the same on a laptop as on a desktop — Trash, iOS backups, caches, old installers, leftovers — but on a small SSD the first two usually finish the job on their own.
Is "Other" storage on a Mac the same as System Data?
Yes. "Other" storage was renamed System Data in macOS Monterey, and the category is unchanged. If you are following a guide that refers to Other storage, or you are on an older macOS that still labels it that way, everything on this page applies without modification.
The rename removed the only useful thing about the old label: "Other" at least admitted it was a leftover bucket. "System Data" reads as though macOS needs all of it, which is why people are afraid to touch a category that is mostly regenerable cache.
Why does System Data keep growing back?
Because most of it is working data that apps recreate. Caches refill as you use your Mac; logs accumulate; snapshots are taken hourly. Clearing System Data is maintenance, not a one-time fix, and a Mac with a healthy amount of it is normal.
What is not normal is System Data that grows and never falls — usually a single runaway application writing logs or caches without limit, or iOS backups nobody has looked at in years.
What about purgeable space?
Purgeable space is storage macOS believes it can reclaim automatically if you run short — mostly local Time Machine snapshots and iCloud files already uploaded. It is counted within System Data. In practice macOS is conservative about reclaiming it, so a Mac can report very little free space while holding tens of gigabytes as purgeable. Thinning snapshots is the reliable way to force the issue.
Frequently asked questions
Is it safe to delete System Data on a Mac?
You cannot delete System Data as a whole, and you should not want to — it includes files macOS needs. What is safe is deleting specific things inside it: caches, old backups, installers and leftovers from deleted apps.
Why is my System Data 100 GB or more?
Almost always old iOS backups, a large browser cache, or virtual machine and container images. Check MobileSync/Backup first — it is the single most common cause of a very large figure.
Does restarting reduce System Data?
Slightly. A restart clears some temporary files and may let macOS purge snapshots, but it does not touch caches, backups or leftovers, which is where the size actually is.
What is the difference between System Data and System?
"System" is macOS itself — roughly 10 to 15 GB and not removable. "System Data" is everything else macOS cannot categorise, and most of it is removable.
Related guides
Sources: Apple — Free up storage space on your Mac · Apple — Back up your iPhone or iPad. Published 30 August 2026. Measurements taken on macOS 26, 994 GB disk.