Is the drive formatted as APFS? What does Get Info on the drive report?I have an old MacBook Pro which needs replacing. It seems to be using most of the 512GB hard drive, so best order the new one with a 1TB SSD.
'About This Mac' says there is 80 GB available and that 206 GB is being used by "System" (rounding the figures here). But Finder reports my usage as 327 GB, but still with only 80 GB free, which doesn't add up to 512.
Some disk space reports exclude purgeable files. For example, MacBooks have Mobile Time Machine automatically available, which uses up free space, but macOS won't report that space as used.
On the other hand, APFS creates challenges in reporting the size of files. Let's say you make a copy of a 1 GB file. On APFS this occupies 1 GB, but the Finder counts it twice, for 2 GB. If you migrate it to a new computer, it would then actually take 2 GB, because the cloning or migration won't retain the APFS blocks.
That gives you one way to get a good estimate of drive size needed: use SuperDuper or Carbon Copy Cloner to clone the MacBook Pro to another drive, then see how much space was used. That's how much space it would initially take on a new computer.