IEEE 1541-2002

IEEE 1541-2002 is a standard issued by the Institute of Electrical and Electronics Engineers (IEEE) concerning the use of prefixes for binary multiples of units of measurement related to digital electronics and computing.

While the International System of Units (SI) defines multiples based on powers of ten (like k = 103, M = 106, etc.), a different definition is sometimes used in computing, based on powers of two (like k = 210, M = 220, etc.) This is due to the use of binary addressing for computer memory locations.

In the early years of computing, there was no significant error in using the same prefix for either quantity (210 = 1024 and 103 = 1000 are equal, to two significant figures). Thus, the SI prefixes were borrowed to indicate nearby binary multiples for these computer-related quantities.

Meanwhile, manufacturers of storage devices, such as hard disks, traditionally used the standard decimal meanings of the prefixes, and decimal multiples are used for transmission rates and processor clock speeds as well. As technology improved, all of these measurements increased. As the binary meaning was extended to higher prefixes, the difference between the two meanings became more pronounced.

This is a common cause of confusion among users that see those amounts reported inconsistently, especially as capacities become bigger and bigger and the absolute error increases. This has even resulted in litigation against hard drive manufacturers, who report drive capacities in standard decimal multiples of bytes, while some operating systems report the size using the smaller binary interpretation of traditional prefixes.

Moreover, there is not a consistent use of the symbols to indicate quantities of bits and bytes — the unit symbol "Mb", for instance, has been widely used for both megabytes and megabits. IEEE 1541 sets new recommendations to represent these quantities and unit symbols unambiguously.

After a trial period of two years, in 2005, IEEE 1541-2002 was elevated to a full-use standard by the IEEE Standards Association, and was reaffirmed on 27 March 2008.