Mac junk files: every kind, measured
Junk on a Mac covers about a dozen kinds of file, each with its own owner, size and rules for removal, and only some of them are safe to delete. On the developer Macs we measured, the largest were temporary build output at 79.6 GB, duplicate files at 30.18 GB, simulator storage at 35 GB (16.9 GB of which no command could reclaim) and AI agent conversations at 22.83 GB. Each kind below has a figure from a real machine, a note on whether the space comes back, and a link to the full measurement.
TL;DR
- Short answer. Junk on a Mac covers about a dozen kinds of file, and only some of them are safe to delete.
- What we measured. The largest on the developer Macs we measured: temporary build output 79.6 GB, duplicates 30.18 GB, simulator storage 35 GB, AI agent conversations 22.83 GB.
- Watch out for. 16.9 GB of that simulator storage couldn’t be reclaimed by any command, so check whether the space really comes back.
AI tools
Agent conversations are the fastest growing kind on a developer Mac, and most of what they hold is not text. We read all 1,231 conversation files in one Codex store, 22.83 GB, and fingerprinted every embedded image: 71% of the bytes were pictures and only 16% were pictures that existed nowhere else. Claude Code keeps the same shape in plain files under ~/.claude/projects, where 610 sessions came to 1.75 GB with about 30% embedded screenshots.
Claude Desktop is different again. Its conversations live on the server, and the 12.6 GB of local data is mostly virtual machine bundles for its computer use feature, 10.9 GB on the Mac we measured. Cursor keeps chat images as ordinary files and a 2.6 GB state database with no images in it, so there is less to reclaim there than the folder size suggests.
Model weights pile up quietly. On one Mac, a single 2.9 GB model file sat in four places at once: an app container, a test fixture, a backup and a support directory. Across the whole home directory, duplicated model weights came to 13.06 GB.
You can remove old conversations you’re done with, and the caches these tools rebuild. Leave alone anything an agent has open right now.
Developer output
Temporary build directories are the largest single pile we have measured. One Mac held 79.6 GB of them under /private/tmp, every one untouched for at least 25 hours, left behind by builds that finished. Xcode DerivedData follows the same rule: the machine can rebuild all of it, and the only cost of deleting it is the next build taking longer.
Simulator storage came to about 35 GB, and this is the kind where size tells you least. 16.9 GB of it could not be reclaimed by any command we tried, including Apple’s own entitled daemon and root. Old Metal toolchains from other Xcode versions add about 670 MB each and can only be removed from one pane inside Xcode.
Agent worktrees are new and growing: 7.17 GB in four worktrees that agents had finished with, beside 10.42 GB in five that agents were still working in. Telling those two piles apart is the hard part, and the only way to do it is to check which processes have files open inside them.
Build output, package caches and worktrees nothing is working in can go. Keep anything with uncommitted work in it, and keep Archives, which hold the symbol files that make a shipped build’s crash reports readable.
System and app data
The Spotlight index for apps reached 12.2 GB on one Mac, with over a hundred of its files held open by the indexing service. It can be rebuilt, but search inside Mail, Notes and Messages stays incomplete until the rebuild finishes, so understand this one before you touch it.
Time Machine keeps local snapshots on your disk between backups, and they can hold tens of gigabytes. macOS thins them automatically when space runs low, which is why a tool that deletes them outright is solving a problem the system already solves.
System Data in Apple’s own storage pane is not a category at all. On the Mac we broke down, Apple’s System Data contained a developer’s temporary files, simulator runtimes and package managers, while its Documents figure contained 68.94 GB of source code. Neither number tells you what to do.
Caches an app rebuilds, logs and the Trash are fine to clear. Leave anything under a folder no process can read, because a size there is a guess, not a measurement.
Your own files, stored twice
Nobody counts this kind, because it doesn’t look like junk. Walking 523,217 files in one home directory found 30.18 GB stored twice, in 44,830 groups of identical files. Under a gigabyte of it was in folders where a cleaning tool could honestly propose deleting something. The rest sat inside app containers and source checkouts.
Copies like these don’t have to be deleted. APFS can let identical files share the same blocks, so both files stay where they are with their own names and contents, and the space comes back anyway.
Old installers are a simpler case of the same thing. A disk image you’ve already installed from is a copy of something you can download again, and these collect in Downloads for years.
How to read a size before you trust it
Two traps make most storage figures wrong, and both inflate them. Cloud placeholders have a name, an icon and a size, and no bytes behind them: one home directory measured 670.0 GB by size and 225.5 GB by what was on disk. Deleting a placeholder frees nothing and takes away your only copy.
The second is sharing. On APFS, two files that share their blocks each report the full size, so adding sizes up promises space that deleting one of them will not give back. The same thing happens with memory, where adding up what each process reports gave 37.04 GB on a Mac using 29.73 GB.
A figure is only worth something if it says what would come back, and a tool should be able to show you afterwards that it did. We hold the numbers on this page to that standard.
Frequently asked questions
What is the biggest kind of junk on a Mac?
On the developer Macs we measured, temporary build output was the largest at 79.6 GB, followed by duplicate files at 30.18 GB and simulator storage at about 35 GB. On a Mac without development tools the answer is usually caches, the Trash, old installers in Downloads and local Time Machine snapshots.
Is it safe to delete cache files on a Mac?
Usually, when the app that owns the cache is not running. A cache exists to be rebuilt. The exceptions are caches that hold something you cannot download again, such as a music app’s offline tracks, which is why they should be listed separately rather than swept up with the rest.
Why does my Mac say System Data is huge?
Because System Data is where Apple’s storage pane puts everything it does not have a category for, including developer files, simulator runtimes and app data. It is a remainder rather than a kind of file, so the figure cannot tell you what to remove.
Do I need an app to clean these up?
No. Every kind on this page can be handled by hand, and each guide gives the commands. An app is useful for finding them, measuring them honestly and keeping track of what it moved, which is the part that takes time.
See which of these are on your Mac
Mint’s scan is free and stays free. It reads your disk on your Mac, lists what it finds with a size that means something, and changes nothing until you choose.