What Is Taking Up Space on My Mac? How to Find Out

You open menu → About This Mac → Storage expecting an answer, and instead you get a bar chart with a huge gray segment labeled “System Data” (sometimes shown as “Other”) that can be 30, 50, even 80+ GB on a Mac that’s been in use for a few years, with no further breakdown offered. Apps, Photos, and Documents are each a reasonable, specific number. System Data is a shrug.

Here’s what’s actually inside that bucket, and how to see it broken down for real.

Why Apple’s Storage view isn’t more specific

Apple’s Storage panel is built for a general audience and deliberately avoids showing anything that looks like a system file a non-technical user might delete by mistake. That’s a reasonable default for the general public, but it means the single largest category on many Macs is also the least explained one. “System Data” isn’t one thing: it’s everything that didn’t cleanly fit into Apps, Photos, Mail, Messages, or Documents, which in practice means:

Every app’s cache in ~/Library/Caches
System and app log files in ~/Library/Logs and /private/var/log
Local Time Machine snapshots
iOS device backups
Xcode’s DerivedData, archives, and simulator devices, if you develop software
Docker’s VM disk image, if installed
Leftover files from apps you uninstalled, see how to uninstall apps on Mac completely
Swap files and other virtual-memory bookkeeping

Individually, several of these are things you can safely clean up. Bundled into one unlabeled number, there’s nothing actionable about it.

How to actually see the breakdown

Terminal gives you a rough pass: du -sh ~/Library/* shows the size of each subfolder inside your Library, which surfaces Caches, Logs, and Application Support as separate numbers instead of one blob. It’s a legitimate way to spot-check, but it’s slow on a full disk scan, doesn’t distinguish safe-to-delete files from files you need, and doesn’t reach into /private/var or other system-level locations without sudo.

Disk Utility shows total used and free space per volume, which confirms the overall number but offers no category breakdown at all. It’s a sanity check, not a diagnostic tool.

A dedicated scanner is the only way to get a genuinely useful breakdown, one built to categorize a full disk scan the way Apple’s own panel doesn’t. OverClean scans your whole disk (or specific folders) and sorts everything into 26 distinct categories, things like media, documents, archives, application caches, developer artifacts, and system data, each with a real size, so “System Data” stops being one number and becomes a list of specific, identifiable things.

The categories that usually explain the mystery

When people run a proper scan after staring at an unexplained System Data figure, the answer is almost always concentrated in a small number of places:

Developer tools, if you write code: node_modules, Python virtual environments, Docker images, and especially Xcode’s DerivedData and simulator devices, which alone commonly reach 15-20+ GB on a Mac used for iOS development.

Caches, across dozens of apps you may not think of as “big” individually but that add up. See how to delete cache on Mac.

Old backups and snapshots: iOS device backups and local Time Machine snapshots, both invisible in Finder unless you know exactly where to look.

Leftover files from uninstalled apps, which have no owner left to account for them and so land squarely in the unexplained bucket.

Together, these four categories account for the overwhelming majority of “System Data” surprises on most Macs.

Why the numbers in About This Mac sometimes don’t add up

A common source of confusion: adding up the categories Apple does show (Apps, Photos, Documents, Mail) often comes nowhere near the total disk usage, with System Data quietly making up the rest of the gap. This is by design rather than a bug: Apple’s categorization is based on file type and known app data locations, and anything it can’t confidently classify falls into the catch-all bucket rather than being guessed at. A general-purpose scan that walks the actual filesystem, rather than relying on Spotlight metadata the way Apple’s panel does, doesn’t have that blind spot, which is why it can attribute space that About This Mac simply can’t.

Purgeable space: the other confusing number

If you’ve looked closely at Disk Utility, you may have also seen a category called “Purgeable”: space macOS considers reclaimable automatically under storage pressure, mainly iCloud-synced files kept locally as a cache and old Time Machine snapshots. macOS is supposed to purge this automatically as needed, but the purgeable figure itself is an estimate, and relying on it to “free itself” rather than checking what’s actually there is a common reason people stay confused about their real usage for longer than necessary.

What a full scan can tell you that Spotlight can’t

Apple’s Storage panel is built on Spotlight’s metadata index, which is fast but incomplete: it’s optimized for search, not exhaustive accounting, and it deliberately skips or under-counts certain locations (deeply nested app support data, hidden dotfiles, some developer tool directories) that a direct filesystem walk picks up without exception. That gap is exactly why two Macs with similar Apps and Photos numbers can show wildly different System Data figures: one of them has years of Xcode or Docker artifacts that Spotlight’s index was never built to surface clearly in a storage summary.

From mystery number to action plan

Once you can see the real breakdown, the fix is usually straightforward: most of what’s in System Data is either safe to delete outright (caches, DerivedData) or safe to review and delete selectively (old backups, unused apps, orphaned leftovers). The full cleanup walkthrough covers the order that gets you the most space back with the least risk.

Download OverClean and run a free scan to see your own breakdown. The Dashboard and category overview cost nothing and need no account. If the scan turns up Applications, Leftovers, or Xcode data you want to dig into further, those deeper views are covered on pricing.