KB or KiB: 1000 or 1024?
There are two ways to count file sizes. Decimal units go up in thousands, like the rest of the metric system: 1 kilobyte (KB) is 1000 bytes and 1 megabyte (MB) is 1000 KB. Binary units go up in 1024s, a power of two that suits computer memory: 1 kibibyte (KiB) is 1024 bytes and 1 mebibyte (MiB) is 1024 KiB.
The gap grows with each step: a KiB is 2.4% bigger than a KB, but a TiB is almost 10% bigger than a TB. The trouble is that many programs show binary sizes with decimal names, so "1 GB" can mean either.
Why does my 1 TB drive show as 931 GB?
Drive makers count in decimal, so a 1 TB drive holds 1,000,000,000,000 bytes. Windows counts in binary but labels the result "GB", so it shows those bytes as 931 GiB, written "931 GB". Nothing is missing. macOS and most phones count in decimal, and show the full 1 TB.
Bits and bytes
A byte is 8 bits. Storage is measured in bytes (B), but network speeds are usually in bits per second
(bit/s), so a 100 Mbit/s connection moves at most 12.5 MB of data a second. A lower-case b
usually means bits and an upper-case B means bytes.