Two meanings of "kilo": decimal and binary
Storage units come in two families that look alike and differ by a growing percentage. The decimal (SI) family multiplies by 1,000: a kilobyte is 1,000 bytes, a megabyte 1,000,000 and so on. The binary (IEC) family, standardised in 1998 to end the ambiguity, multiplies by 1,024 and uses names like kibibyte (KiB), mebibyte (MiB), gibibyte (GiB) and tebibyte (TiB). Memory chips are naturally sized in powers of two, so RAM is binary, while drive makers and network gear sell in decimal units.
This converter holds bit, kilobit, megabit, gigabit, byte, kB, MB, GB, TB, PB, KiB, MiB, GiB and TiB against the byte (one bit is 0.125 B). The decimal 1,000-byte kilobyte is labelled kB, as strict SI writes it, though people also search for it as KB. The bit units are decimal too: a megabit is 1,000,000 bits, or 125,000 bytes, so a 100 Mbit figure converts straight to 12.5 MB.
Why a "1 TB" drive shows about 931 GiB
A manufacturer's 1 TB is 1,000,000,000,000 bytes. Windows historically divides by 1,024³ and labels the result "GB", although the number is gibibytes, giving 1,000,000,000,000 / 1,073,741,824 = 931.32. Nothing is missing from the drive. Only the unit changed, and the gap grows with scale.
| Prefix step | Binary / decimal | Gap |
|---|---|---|
| KiB vs kB | 1,024 / 1,000 | 2.4% |
| MiB vs MB | 1,048,576 / 1,000,000 | 4.9% |
| GiB vs GB | 1,073,741,824 / 109 | 7.4% |
| TiB vs TB | 1,099,511,627,776 / 1012 | 10.0% |
More worked examples
- 256 GB (decimal) is 238.42 GiB. A 2 TB drive is 1,862.6 GiB, or 1.819 TiB.
- 1,024 MB is only 1.024 GB, but 1,024 MiB is exactly 1 GiB.
- A 4.7 GB DVD holds about 4.377 GiB.
- 16 GiB of RAM is 17.18 GB in decimal terms.
Conventions and mistakes
- macOS and many Linux tools display decimal units, Windows historically shows binary values with decimal labels, and some tools use the IEC names. Check which a tool uses before comparing.
- Bytes are 8 bits, so a "100 Mbps" line moves at most 12.5 MB per second, before protocol overhead.
- For non-storage number systems such as hex and binary, the number base converter is the right tool, and to shrink a file rather than measure it, see the image compressor.