Where did my Mac storage go, and how to actually find out
macOS tells you storage is full but not what is filling it. Here is how to find the real occupants of your drive, in the order that reclaims the most space.
- Storage
- Beginner
- System Data
The macOS storage bar tells you the disk is full and then declines to say why. Its categories are so broad as to be useless: a 60 GB block labelled System Data is not an answer, it is a shrug. Meanwhile the actual occupant is usually three or four specific folders, and once you can see them the problem is often twenty minutes of work rather than an afternoon.
Why the built-in screen cannot answer this
Apple's storage screen groups by what a file is for, not by where it is or how big it got. Two problems follow. Anything it cannot classify falls into System Data, which is why that bar is enormous on almost every machine. And a category total tells you nothing actionable: knowing Documents is 80 GB does not tell you which document.
What you want instead is the disk laid out by size, so the biggest things are the most visible things. That is what a treemap does, and it is why the first look at one is usually surprising.
The four places the space usually is
In rough order of how much they typically hold on a machine that is out of space:
| Where | Typical size | Safe to clear? |
|---|---|---|
| Developer caches and build output | 20 to 150 GB on a dev machine | Yes, all of it regenerates |
| Application support files and caches | 5 to 40 GB | Mostly, and entirely for apps you removed |
| Old downloads and disk images | 5 to 30 GB | Yes, check before removing |
| iOS backups and simulator runtimes | 10 to 60 GB if you have ever used Xcode | Yes, unless a backup is your only copy |
Doing it in order
- Look at the whole disk as a map first. Do not start deleting. Spend two minutes seeing what is actually there, because the intuition about which folder is the problem is wrong about half the time.
- Start with things that regenerate. Build output and caches are free to delete in the real sense: the machine rebuilds them on demand. Nothing is lost and nothing needs a decision.
- Then check apps you have already removed. Dragging an app to the Trash leaves its support files behind, sometimes gigabytes of them, belonging to software that is not on the machine any more.
- Then the large individual files. Old installers, video exports, virtual machine images. These need judgement, which is why they come last.
The number that surprises people
On a machine used for software development, the single largest reclaimable category is almost always build artefacts: node_modules folders in projects you have not opened in a year, Xcode DerivedData, Docker images, simulator runtimes. It is routine for this to be 50 GB and not unusual for it to pass 150 GB. None of it is precious and all of it regenerates. If you write software, start there.
If you do not, the equivalent is usually application support files, especially from apps you have already uninstalled. That is its own problem, and the reason a Mac that has had many apps come and go feels heavier than a fresh one.
What to be careful with
Two rules make this safe. First, prefer moving things to the Trash over deleting them outright, so a mistake costs a restore rather than a backup. Second, do not delete anything inside /System or /Library that you cannot name. The reclaimable space is almost never there, so the risk buys you nothing.
Common questions
Why is System Data so large on my Mac?
System Data is the category macOS uses for everything it cannot sort into another one, so it collects caches, logs, developer build output, virtual machine images and more. A large System Data figure does not indicate a problem with macOS itself. It means several unrelated things are being totalled together, and you need a tool that shows the actual folders to find out which.
Is it safe to delete cache files on a Mac?
Application caches are safe to remove. They exist to make things faster on second use, and the app rebuilds what it needs. The visible cost is that a few apps take slightly longer to open the next time. Avoid deleting anything inside the System folder, where the reclaimable space is small and the risk is not.
How much free disk space should a Mac have?
Aim to keep at least 10 to 15 percent of the drive free. macOS uses free space for virtual memory, temporary files and updates, and performance degrades noticeably as a drive approaches full. On a 512 GB drive that means keeping roughly 50 to 75 GB available.