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“Your startup disk is almost full” — how to fix it properly

The warning usually arrives at the worst moment: mid-export, mid-update, mid-deadline. macOS needs free space to breathe — for swap, for temporary files, for updates — and once you drop below roughly 10% of your volume, performance degrades and apps start failing to save. Here is the sequence that gets space back fastest, ordered by how much each step typically returns.

Updated 2026-08-02 · Written by the CleanerMac team · macOS 13–15 tested

First, get an accurate picture

Before deleting anything, find out what you are actually dealing with. Open Terminal and run df -h /. Look at the Avail column — that is the truth. The Storage pane in System Settings caches its figures and often disagrees.

Then run du -sh ~/* | sort -h and du -sh ~/Library/* | sort -h. Between them, these two commands identify the cause on most Macs in under two minutes. Ignore the permission-denied lines; they are expected.

Finally, open Apple menu → System Settings → General → Storage for the category breakdown and Apple's own built-in recommendations (Store in iCloud, Optimise Storage, Empty Trash Automatically, Review Files). Those four are genuinely useful and cost nothing.

Step 1 — Thin Time Machine local snapshots

This is the highest-yield step and the one most people have never heard of. When your Time Machine drive is not connected, macOS still takes hourly backups and stores them as APFS snapshots on your startup disk. They accumulate silently.

See what you have: tmutil listlocalsnapshots /

Reclaim it: sudo tmutil thinlocalsnapshots / 999999999999 4

The large number is a byte target — effectively “as much as possible” — and 4 is the highest urgency level, which tells macOS to delete rather than defer. Your real backups on the external drive are completely unaffected. Re-run df -h / afterwards to see the change.

Step 2 — Empty the Trash, including per-app Trash

Obvious but frequently skipped. Also check inside Photos (recently deleted keeps items for 30 days), Mail (Trash mailbox), and any app with its own bin. In Photos: select Recently Deleted in the sidebar, unlock it, then choose Delete All.

Step 3 — Delete old iOS and iPadOS backups

Look in ~/Library/Application Support/MobileSync/Backup. Each subfolder is one device backup and can easily exceed 20 GB. Check modification dates to identify old ones. You can also manage them through Finder with the device connected, or via Apple menu → System Settings → General → Storage → iOS Files.

Step 4 — Clear caches

Quit the app first, then empty the contents of its folder inside ~/Library/Caches — not the folder itself. Browser caches are usually the largest; each browser also has a built-in Clear Browsing Data option that is safer than deleting files by hand. Our full walkthrough is at how to clear cache on Mac.

Step 5 — Hunt down large files

Use Finder's search: Command-F, then set the criteria to File Size is greater than 1 GB, and change the scope to This Mac. Or in Terminal, find ~ -type f -size +1G 2>/dev/null. Video files, virtual machine images, old project archives and forgotten downloads dominate this list. Our guide on finding large files on Mac goes deeper, including how to catch files you have not opened in a year.

Step 6 — Uninstall apps properly

Dragging an app to the Trash removes the bundle and leaves its caches, containers, preferences and Application Support data behind — sometimes gigabytes per app. Do it properly using our guide to uninstalling apps on Mac, and be especially thorough with Adobe apps, Docker, Xcode and anything that shipped its own installer.

How much space each step typically returns

Emergency measures when you cannot even boot properly

If the disk is so full that macOS will not let you log in normally or apps refuse to launch:

1. Boot into Safe Mode (Apple silicon: hold the power button until startup options appear, select your disk, hold Shift and click Continue in Safe Mode). Safe Mode clears some system caches automatically.
2. From there, run the snapshot thinning command in Terminal — it works in Safe Mode and needs almost no free space to operate.
3. If you cannot log in at all, boot to Recovery, open Terminal from the Utilities menu, and run tmutil thinlocalsnapshots /Volumes/<your data volume> 999999999999 4 against the data volume (check its exact name with ls /Volumes) — run ls /Volumes first and substitute your volume's real name, since it is only “Macintosh HD - Data” on a default install.
4. As a last resort, move large personal folders to an external drive to get back above the threshold, then clean up properly once the system is usable.

Stopping it from happening again

Keep at least 10–15% of the volume free as a working target. Connect your Time Machine drive regularly so snapshots flush instead of accumulating. Turn on Empty Trash Automatically in the Storage settings. Enable Optimise Mac Storage for Photos if you use iCloud Photos. Clear Downloads as a monthly habit. And if your Mac has also started feeling sluggish, a full disk is a common cause — see how to speed up a slow Mac.

For people who would rather not repeat this sequence by hand every few months, CleanerMac automates the whole pass: it finds the snapshots, caches, backups, large files and app leftovers in one scan, explains what each item is, and routes deletions of your own files through the Trash so those decisions can be undone. One purchase, no subscription, and nothing leaves your Mac.

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