Convert data units between units, using exact SI factors.
Both are in use, which is exactly where the confusion comes from. SI prefixes, as set out in the BIPM brochure, mean powers of 1000, so a kilobyte is 1,000 bytes. Binary counting in 1024s has its own prefixes — kibibyte, mebibyte, gibibyte. This calculator shows both.
Manufacturers state capacity in SI units, where a terabyte is 1000⁴ bytes, while the operating system often reports it in binary units, where a tebibyte is 1024⁴. The gap is about 9%, so a 1 TB drive shows as roughly 931 GB. Neither side is lying — they are using different units.
A bit is a single binary digit, zero or one; a byte is eight bits. Internet speed is measured in bits per second but file size in bytes, so a 100 Mbit/s connection downloads about 12.5 MB per second. That factor of eight is among the most common misunderstandings, and both units take their prefixes from the same SI system.
A kibibyte is 1,024 bytes, introduced precisely so that binary counting would stop colliding with SI prefixes, where kilo always means a thousand. It is written KiB, not KB. In practice it is rarely used, and that inconsistency is what keeps the drive-capacity confusion alive.