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A binary prefix is a unit prefix for multiples of units in data processing, data transmission, and digital information, notably the bit and the byte, to indicate multiplication by a power of 2.
The computer industry has historically used the units kilobyte, megabyte, and gigabyte, and the corresponding symbols KB, MB, and GB, in at least two slightly different measurement systems. In citations of main memory (RAM) capacity, gigabyte customarily means 1073741824 bytes. As this is the third power of 1024, and 1024 is a power of two (210), this usage is referred to as a binary measurement.
In most other contexts, the industry uses the multipliers kilo, mega, giga, etc., in a manner consistent with their meaning in the International System of Units (SI), namely as powers of 1000. For example, a 500 gigabyte hard disk holds 500000000000 bytes, and a 1 Gbit/s (gigabit-per-second) Ethernet connection transfers data at 1000000000 bit/s. In contrast with the binary prefix usage, this use is described as a decimal prefix, as 1000 is a power of 10 (103).
The use of the same unit prefixes with two different meanings has caused confusion. Starting around 1998, the International Electrotechnical Commission (IEC) and several other standards and trade organizations addressed the ambiguity by publishing standards and recommendations for a set of binary prefixes that refer exclusively to powers of 1024. Accordingly, the US National Institute of Standards and Technology (NIST) requires that SI prefixes only be used in the decimal sense:kilobyte and megabyte denote one thousand bytes and one million bytes respectively (consistent with SI), while new terms such as kibibyte, mebibyte and gibibyte, having the symbols KiB, MiB, and GiB, denote 1024 bytes, 1048576 bytes, and 1073741824 bytes, respectively. In 2008, the IEC prefixes were incorporated into the ISO/IEC 80000 standard.
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