Mac storage basics · Apps and macOS itself
How much free space do I need to install a macOS update?
Short answer: Several times the download size while it unpacks and prepares; Software Update says how much more it needs. Free space by emptying the Trash, releasing snapshots and moving big folders off.
More than the download size. An update has to download, unpack and prepare itself, and macOS usually makes a snapshot first, so a major upgrade can need several times the download size free while it installs. Software Update tells you how much more space it needs if there isn't enough.
To make room quickly: empty the Trash, release local snapshots, delete old installers and device backups, and move big folders or apps to an external drive. Storage Studio's Local Snapshots window can reclaim a specific amount (for example the amount Software Update asked for). See the Local Snapshots manager.