What is System Data on a Mac?
System Data is macOS's catch-all storage category for files that do not fit another label. Here is what it can contain, why the number changes, and how to reduce the parts you can identify 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.
It can include app and system caches, log files, old iOS device backups, disk images sitting in Downloads, virtual machine images and support folders belonging to apps you deleted long ago. Depending on the Mac and its backup settings, local Time Machine snapshots may be included too.
System Data does not have a fixed healthy size. The number can change after an update, restart, backup or normal app use, and some of it may be reclaimed by macOS when space is needed. Apple describes System Data (or “Other” on older versions) as the general category for Apple and third-party files that do not belong to a more specific category. On one Mac we measured 84.8 GB of removable files, about 8.5% of a 994 GB disk; that is a measured example, not a normal target for every Mac.
System Data is a category, not a folder. Some items in it are temporary and removable; others are active app data, backups or files macOS needs. Identify the underlying bucket before moving anything to the Trash. See Apple's storage guidance for the built-in ways to free space.
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. The table above is from a developer Mac — dependency folders and package caches dominate. On a consumer MacBook Air the same grey bar is more often browser caches, iOS backups and Messages attachments.
Is a large System Data number normal?
A large number by itself does not tell you whether anything is wrong. System Data is recalculated by macOS and varies with the Mac's storage size, installed apps, local iPhone backups, developer tools and Time Machine settings. A better test is whether the Mac has enough free space for your work and whether one identifiable bucket is growing unusually fast.
- Changing after a restart or update: often a recalculation or temporary system data.
- Large and stable: look for old backups, caches, installers or developer data.
- Growing continuously: find the app or folder writing the data before deleting anything.
See inside your System Data free. Broomkit maps reclaimable categories on your Mac — no account, no card to download.
Download FreeHow Broomkit categories map to System Data
When people ask what is “inside” System Data, they want named buckets they can act on. These Broomkit categories are the ones that most often feed the grey bar:
| Broomkit category | Why it lands in System Data | Usually safe? |
|---|---|---|
| App caches | Temporary files apps never tidy | Yes |
| Package caches | Installer / package-manager leftovers | Yes — re-downloaded on demand |
| System caches | User-level macOS caches | Yes in ~/Library/Caches |
| Old installers | DMG/PKG files left in Downloads | Yes after install |
| App leftovers | Support files from deleted apps | Yes if the app is gone |
| Logs | Diagnostic output | Yes for old logs |
| iOS backups | Local device backups | Yes if iCloud Backup covers you |
| Simulators / Docker | Dev runtimes and disk images | Yes with the right prune command |
How to reduce System Data on a Mac
You cannot delete a “System Data” folder. Reduce the grey bar by
clearing regenerable caches, removing old iOS backups,
uninstalling leftover support files,
deleting stale installers, and thinning local Time Machine snapshots — never
by wiping /System or Containers wholesale. The step-by-step order is in
how to delete System Data on a Mac safely.
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. The walkthrough on uninstalling an app without leaving leftovers shows where they hide.
- Thin Time Machine local snapshots with
tmutil thinlocalsnapshots / 21474836480 4in Terminal. This needs an administrator password.
For a step-by-step walkthrough of those buckets — including what never to delete — see how to delete System Data on a Mac safely. On the reference Mac these categories summed to 84.8 GB of removable files inside what Storage labelled System Data.
iOS and iPad backups on a Mac
Finder (and older iTunes) can store full device backups under
~/Library/Application Support/MobileSync/Backup. Each backup is a folder
of opaque files; sizes of 20–100+ GB are common. If the same device also uses iCloud
Backup, the local copy is often redundant. Confirm the device still backs up to
iCloud before you remove a local backup, then move the dated folder to the Trash —
do not delete random files inside a backup you still need.
Free for 3 days. See which System Data categories are reclaimable on your Mac, and clean them, during the trial.
Download Free for MacMacBook 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 — same grey bucket, same mix of caches, logs, backups and leftovers.
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. In practice you still reduce it by clearing safe user caches and old backups, not by deleting the operating system itself. Apple documents free-space steps in Free up storage space on your Mac.
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.
Purgeable is also why “About This Mac” free space and Finder free space sometimes disagree with Storage settings. Treat System Data as a classification, not a folder you can open and empty in one click — reduce the named buckets instead.
When a Mac cleaner helps with System Data
Apple’s Storage UI will not list the caches and leftovers that make up most of the grey bar. A transparent cleaner that shows paths is a practical way to see which regenerable files you are about to remove. Avoid tools that claim to “delete System Data” as a single action — that wording usually means a blind sweep. Prefer a scan that separates safe caches from review-only items, and that uses the Trash. More on choosing a tool: CleanMyMac alternatives and how to free up disk space on a Mac.
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 and should not be deleted. "System Data" is a broad category for Apple and third-party files that do not fit another label; some parts are removable, but active app data and other system-managed files are not junk.
Sources: Apple — Free up storage space on your Mac · Apple — Back up your iPhone or iPad. Published 30 August 2026 · Updated 12 September 2026. Measurements taken on macOS 26, 994 GB disk.