Why your Mac says there isn’t enough space
A macOS update has to download, unpack and then install, all on your startup disk. For a while it needs room for the old system, the new system and the files in between. If that room isn’t there, Software Update stops before it starts.
When the disk looks fine but the update still fails
This catches most people out. Storage settings shows available space, which includes purgeable files that macOS promises to delete when needed, such as Time Machine snapshots and iCloud Drive files stored in the cloud. The installer doesn’t always wait for macOS to purge them, so it sees less room than you do.
To see the real figure, click Macintosh HD in the Finder sidebar and press Command-I. If the purgeable number in brackets is large, start with fix 4 below. Our guide on clearing purgeable space on a Mac explains it in full.
How much space does a macOS update need?
There is no single number. Read the message in System Settings, then General, then Software Update. It states how much space your Mac needs for that update.
- Point updates, such as a security fix, usually need a few gigabytes.
- Major yearly upgrades usually ask for tens of gigabytes, because they replace the whole system.
Aim to free more than the message asks for. A buffer of 10 to 20 GB on top helps the install finish without a second warning halfway through, and leaves macOS room for swap files once you restart.
What the different messages mean
The wording changes depending on where the update stops. Each version points to a slightly different fix.
| Message or symptom | What it means | Start with |
|---|---|---|
| Requires X GB of available storage | There isn't enough room to download and prepare | Fixes 1 to 5 |
| Not enough free space, yet Storage looks fine | Purgeable space hasn't been released | Fix 4, then a restart |
| Download finishes, install fails | No room left to unpack the update | Free 10 to 20 GB more |
| Upgrade missing from Software Update | Your Mac may not support it | Check Apple's compatibility list |
If the figure Software Update asks for keeps rising as you free space, that is normal. It measures the gap between what you have and what it needs, and recalculates whenever it checks again.
9 ways to free space for a macOS update
Work from the top. Each fix is safe, and the first few free the most space on most Macs. Check the Software Update figure after each one, and stop when you have enough.
The quickest big wins
| What to remove | Where to find it | Typical size |
|---|---|---|
| Old macOS installers | Install macOS ….app in Applications | Often over 10 GB each |
| iPhone and iPad backups | Storage settings, iOS Files | Often tens of GB |
| Downloads folder | ~/Downloads | Varies, often large |
| Time Machine snapshots | Disk Utility, Show APFS Snapshots | Varies, often large |
| App and browser caches | ~/Library/Caches | Usually a few GB |
| Developer files | ~/Library/Developer | Can reach tens of GB |
1. Empty the Trash
Files in the Trash still use disk space. Right-click the Trash in the Dock and choose Empty Trash. It sounds obvious, yet it is often the reason the earlier cleanup didn’t help.
2. Delete old macOS installers
Open Applications and look for apps named Install macOS followed by a version name. If an older installer is still there from a past upgrade, it can take over 10 GB. Move it to the Trash. You can download it again from Apple if you ever need it.
3. Remove old iPhone and iPad backups
Open System Settings, then General, then Storage, and click the info button next to iOS Files. Delete backups of devices you no longer own. Our guide on deleting iPhone backups on a Mac has every step.
4. Clear Time Machine local snapshots
Open Disk Utility, choose View, then Show APFS Snapshots, and delete the ones named com.apple.TimeMachine. This turns purgeable space into free space straight away. Leave any snapshot named com.apple.os.update in place.
5. Sort out your Downloads folder
Open Downloads in Finder, switch to list view and click the Size column. Old disk images, zip files and installers you already used are safe to remove.
6. Clear app and browser caches
Caches rebuild as you use your apps, so clearing them is safe and fast. Our guide on how to clear cache on a Mac shows which folders to empty.
7. Find large files and duplicates
Old video exports, virtual machines and copies of the same photo library add up quickly. Use our guides to find large files and find duplicate files on a Mac.
8. Move big files to an external drive
You can’t install the update on an external drive, but you can move large files there. Copy video projects, music libraries or old archives to the drive, check they open, then delete the originals from your Mac.
9. Clean up developer files
If you write code, check ~/Library/Developer. Xcode build data and old simulators often grow into tens of gigabytes. Our guide on Xcode taking up too much space shows what is safe to remove.
Still not enough space? Try these
Restart and try again
A restart clears temporary files and gives macOS a chance to purge what it promised to. Open Software Update straight after you log in, before other apps start filling caches again.
Turn on Optimise Mac Storage
Open System Settings, click your name, then iCloud, then Drive, and turn on Optimise Mac Storage. Older files stay in iCloud and download when you open them. Photos has a similar option in its own settings.
Check the update isn’t stuck on a partial download
If a previous attempt failed halfway, the partial download may still sit on the disk. Restart and open Software Update again. macOS usually reuses or clears it on its own, so avoid deleting system folders by hand.
Make sure your Mac supports the update
If you can’t see the upgrade at all, your Mac may not support it. Apple lists compatible models on each macOS release page. No amount of free space changes that.
Before you click Update Now
- Back up with Time Machine or another tool. A major update rarely goes wrong, but it can.
- Plug a MacBook into power and keep it on the charger until the update finishes.
- Leave the extra buffer of free space in place until the first restart is done.
Don’t refill the disk while the update downloads. Large file copies, video exports and new app installs all compete for the same space, and the installer can run short at the last step. Let it finish, restart, and then move your files back if you parked any on an external drive.
Once the update is in, check Storage settings again. The figure often drops a little, because macOS removes the installer and temporary files it no longer needs.
Keep room for the next update
Apple releases a major macOS version every year and point updates in between. Keeping 10 to 15 percent of your disk free means you won’t hit this message again. Our guide on how to free up space on a Mac lists 14 habits that help.
MyMacStorage makes the check quick. One scan shows what fills your disk, and you clear it to the Trash with Undo ready. It costs $14.99 once, with no subscription. Learn more on the home page.