Why does formatting decrease storage capacity

Apple notes:

_{1GB = 1 billion bytes; actual formatted capacity less.}

Why is the actual formatted capacity less than the unformatted capacity?

Solution 1:

Formatting here really means partitioning and creating file system(s).

Depending on the file system used, a more or less extended part of the media will be used to store non data stuff.

This includes tables used to identify used and unused areas, the file system hierarchy, journals, file metadata (owner, permissions, timestamps, acls), checksums, etc. Some space is also lost due the record size used (a one byte file will use more than one byte on disk).

Not to mention the confusion created by the normalization of the GB unit, which used to mean 1073741824 bytes (i.e. 1024x1024x1024 = 2^30) for decades but now should (rightly) mean 1000000000 bytes (i.e. 1000x1000x1000).