TidyMacShort answer
A macOS update usually needs 20–40 GB free. The fastest safe wins, in order: thin Time Machine local snapshots, delete old installers and “Install macOS” apps, remove old iPhone backups, clear caches of closed apps, and empty the Trash. Then restart and try the update again.
The steps
- Thin Time Machine local snapshots.
- Delete old installers and “Install macOS” apps.
- Remove iPhone backups you don’t need.
- Clear caches of closed apps.
- Empty the Trash and restart.
How much space you need
Apple's installer needs room to download the update and unpack it. Plan on 20–40 GB free for a major version and 5–15 GB for a minor one.
The order that recovers space fastest
- Snapshots: often 10–50 GB and invisible in Finder. Thin them
- Old installers: an old "Install macOS" app is 12–15 GB. Find them
- Device backups: 5–20 GB each. Check first
- App caches: a few GB. Safe for closed apps
- Empty the Trash, then restart.
Let TidyMac do this for you
Scan free, see every part of your storage explained, and clean only what’s safe. Everything goes to the Trash first.
If the disk is completely full
macOS can behave oddly with almost no free space. Start with old installers in Downloads; deleting a single disk image is usually enough to let everything else run.