Mac storage full: where the gigabytes actually hide
"Your disk is almost full" — and Finder shows you nothing. The twelve places the space really sits, how large each one typically gets, and how much of it comes back for free.
The warning always arrives at the wrong moment: "Your disk is almost full." You open Finder, find a home folder holding maybe 40 GB — and System Settings insists that 900 of 1000 GB are in use. The difference lives in places Finder does not show by default.
This is the map of those places, in the order they pay off on a working machine.
Why "System Data" is so large
Apple calls the category System Data, and it is not a category but a remainder: everything macOS could not file anywhere else. It regularly contains:
- Application caches under
~/Library/Cachesand/Library/Caches - Sandboxed app containers under
~/Library/Containers - Local APFS snapshots that Time Machine parks on the internal disk
- Logs and diagnostic reports under
~/Library/Logs - Developer leftovers, if Xcode or Docker was ever installed
None of these show up when you search your home folder for large files. That is why a full Mac feels like a riddle — the data is not hidden, it is simply not where anyone looks.
The twelve places, ordered by yield
| Place | Typical | Does it come back? |
|---|---|---|
~/Library/Developer/Xcode/DerivedData | 10–60 GB | entirely, rebuilt on the next build |
| Docker (images, volumes, build cache) | 5–80 GB | partly — volumes can be data |
~/Library/Developer/CoreSimulator/Devices | 5–40 GB | per simulator, each with its own data |
~/Library/Developer/Xcode/iOS DeviceSupport | 3–25 GB | entirely, re-downloaded when needed |
~/.npm, ~/.pub-cache, ~/Library/Caches/pip | 2–15 GB | entirely, purely re-downloadable |
Homebrew downloads (brew cleanup -n) | 1–10 GB | entirely; Homebrew names the figure itself |
~/Downloads | 2–40 GB | your call — usually installers from 2023 |
~/Library/Mail (attachments) | 1–30 GB | through Mail's settings, not through Finder |
| iPhone and iPad backups | 5–150 GB | only if a copy exists elsewhere |
| Photos library (originals kept local) | 10–400 GB | through iCloud optimisation, not by deleting |
| Bin | 0–50 GB | instantly, and routinely forgotten |
| Leftovers of removed applications | 0.5–8 GB | entirely, once they have been matched |
The order matters: on a developer machine the first row often yields more than the next five combined, and it costs exactly nothing — the next build puts it all back.
The mistake cleaning software makes
A cleaner sees a 41 GB file called Docker.raw and offers to empty it. What it does not see is that four separate accounts live inside, and one of them is named volumes — the database somebody created for a project a year ago.
du can measure how large a folder is. Only the tool that owns the folder knows how much of it comes back for free. So BalaneDisk asks the tool:
docker system df -v
brew cleanup -n
xcrun simctl list -jEvery one of those calls is read-only, and each answers the question a folder size leaves open.
What ten minutes actually gets you
A realistic order for a Mac that is full:
- 01Empty the Bin. Free, instant, permanently forgotten.
- 02
~/Library/Developer/Xcode/DerivedData— if present, usually the single largest item. - 03Clear Docker in stages, never in one sweep: build cache first, then unused images, volumes last and one at a time.
- 04Review the simulators. A device you have not booted in two iOS releases still holds its full data set.
- 05Package manager caches through their own commands.
- 06Go through Downloads. Installers, DMGs and ZIPs of things that are long since installed.
After that a typical developer Mac is 60 to 120 GB lighter without a single document of yours being touched.
Why we still delete nothing
BalaneDisk shows each of these findings with its size, its verdict and the exact route — the command the owning tool wants, or the folder opened in Finder. Nothing is deleted. Software that empties folders unasked is asking for trust with data it does not understand; the decision, and the undo, stay with you.
More on that: Why BalaneDisk never deletes anything.
- Why does "About This Mac" show more used space than my folders add up to?
- Because most of it sits in hidden folders under ~/Library, in APFS snapshots and in system data that Finder does not display. The "System Data" category is not a category at all — it is the remainder macOS could not file anywhere else.
- Can I just delete ~/Library/Caches?
- Technically yes, usefully rarely. Most applications rebuild their cache immediately; some lose signed-in sessions or local drafts on the way. Going after the big ones specifically — Xcode, Docker, npm, through their own commands — returns more space and costs nothing.
- How much free space should an SSD keep?
- Ten to fifteen percent as a rule of thumb. Below that, APFS gets noticeably slower at creating snapshots and writing large files, and some applications abort a save instead of waiting.