In telecommunications, bit rate or data transfer rate is the average number of bits, characters, or blocks per unit time passing between equipment in a data transmission system. This is typically measured in multiples of the unit bit per second or byte per second.

To be as explicit as possible, both the prefix and the suffix of the unit must be known. For example, the abbreviation 2 Mb can actually be expanded in 4 different ways (mega- vs mebi- and -bit vs -byte). The difference in the associated numbers can be significant:

The table above shows an approximate 5% difference between the corresponding mega- and mebi- units with a 800% difference between -bit and -byte units. Explicitness in units is important because difference can become even larger across different prefix units.

k- stands for kilo, meaning 1,000, while Ki- stands for kilobinary ("kibi-"), meaning 1,024. The standardized binary prefixes such as Ki- were relatively recently introduced and still face low adoption. K- is often used to mean 1,024, especially in KB, the kilobyte.