macOS updates need free space to download the update, unpack it and install it, and a major upgrade needs considerably more than the download size suggests. If your disk is too full, Software Update tells you it cannot continue.
Find out how much space you need
Choose Apple menu > System Settings > General > Software Update. If there is not enough space, the message usually says how much more is needed. Note that number; it is your target.
Then check your current free space in System Settings > General > Storage.
Quick wins, in order
Work through these until you have comfortably more than the space required.
1. Empty the Trash
Right-click the Trash in the Dock and choose Empty Trash. Files in the Trash still use space.
2. Delete old macOS installers
If you previously downloaded a full macOS installer, it may still be in Applications as an app named Install macOS .... These are large and can be deleted. Also check Downloads for old installers and disk images (.dmg, .pkg).
3. Remove old device backups
In Storage, click ⓘ next to iOS Files and delete backups you no longer need. See deleting old iPhone backups on a Mac.
4. Remove large files and apps
In Storage, click ⓘ next to Documents > Large Files and Applications to find the biggest items. Move large files you want to keep to an external drive. See finding large files on a Mac.
5. Clear big caches and developer storage
Browsers, creative apps and developer tools can keep many gigabytes of caches. See clearing the cache on a Mac. If you develop software, clearing Xcode DerivedData and simulators often recovers the most.
6. Restart
Restarting clears swap files and temporary files and lets macOS recalculate free space. Check Storage again after restarting.
Temporarily move files to an external drive
If you are still short, copy large folders such as Movies or a big project to an external drive, check the copy opened correctly, and then delete them from your Mac. Copy them back after the update if you want.
Use iCloud to free space
If you have spare iCloud storage, Store in iCloud in Storage settings can move your Desktop, Documents and Photos originals to iCloud, freeing space quickly. Downloads may take a while to finish before the space is released. See using iCloud to free up space.
During the update
Keep your Mac plugged in and do not close the lid or let it sleep. The installer may restart several times and show a blank screen or progress bar for a while. That is normal.
Frequently asked questions
How much space do I need to update macOS?
It depends on the update and your Mac. Minor updates need a few gigabytes; major upgrades can need many tens of gigabytes. Software Update tells you how much more you need if there is not enough.
Can I delete the Install macOS app after upgrading?
Yes. Once your Mac is upgraded, the installer app in Applications is not needed. You can download it again later from Apple if you need it.
Why does my Mac still say not enough space after I deleted files?
The files may still be in the Trash, or macOS may need a restart to update its free space figure. Empty the Trash and restart, then try again.
Want this done for you?
spacerecover scans your Mac for caches, old installers, app leftovers, large files and developer build-up, explains each one in plain words, and clears what you approve in one step. Items go to the Trash first, so you can undo.