Mac storage basics · Checking your storage
Why does my 256 GB Mac show less than 256 GB of total storage?
Short answer: A small part of the drive is reserved for macOS Recovery and other hidden system areas, so Macintosh HD reports slightly less than the advertised size. It's normal and doesn't grow.
Part of the drive is set aside for things macOS keeps outside your main volume — the Recovery system, firmware-related areas on Apple silicon, and the space used for virtual memory and sleep — so the capacity Finder reports for Macintosh HD is a little less than the number on the box.
Apple and drive makers both count 1 GB as 1,000,000,000 bytes, so it isn't a units trick on a Mac; the difference is genuinely reserved space you can't use for files. It is not a fault and it doesn't grow over time.
What does grow is System Data and purgeable space inside your volume — see what System Data is.