The kilobyte is a multiple of the unit
byte for
digital information.
The
International System of Units (SI) defines the prefix kilo as a multiplication factor of 1000 (103); therefore, one kilobyte is 1000 bytes.[1] The internationally recommended unit symbol for the kilobyte is kB.[1]
In some areas of
information technology, particularly in reference to
random-access memory capacity, kilobyte instead typically refers to 1024 (210) bytes. This arises from the prevalence of sizes that are
powers of two in modern digital memory architectures, coupled with the coincidence that 210 differs from 103 by less than 2.5%. A
kibibyte is 1024 bytes.[1]
The international standard
IEC 80000-13 uses the term "byte" to mean eight
bits (1 B = 8 bit). Therefore, 1 kB = 8000 bit. One thousand kilobytes (1000 kB) is equal to one
megabyte (1 MB), where 1 MB is one million bytes.
Binary (1024 bytes)
The term 'kilobyte' has traditionally been used to refer to 1024 bytes (210 B).[5][6][7] The usage of the metric prefix kilo for binary multiples arose as a convenience, because 1024 is approximately 1000.[8]
The binary interpretation of metric prefixes is still prominently used by the
Microsoft Windows operating system.[9] Binary interpretation is also used for
random-access memory capacity, such as main memory and
CPU cache size, due to the prevalent
binary addressing of memory.
The binary meaning of the kilobyte for 1024 bytes typically uses the symbol KB, with an uppercase letter K. The B is sometimes omitted in informal use. For example, a processor with 65,536 bytes of cache memory might be said to have "64 K" of cache. In this convention, one thousand and twenty-four kilobytes (1024 KB) is equal to one megabyte (1 MB), where 1 MB is 10242 bytes.
In December 1998, the
IEC addressed such multiple usages and definitions by creating prefixes such as kibi, mebi, gibi, etc., to unambiguously denote powers of 1024.[10] Thus the kibibyte, symbol KiB, represents 210 bytes = 1024 bytes. These prefixes are now part of IEC 80000-13. The IEC further specified that the kilobyte should only be used to refer to 1000 bytes. The
International System of Units restricts the use of the SI prefixes strictly to powers of 10.[11]
Use of term
The
Shugart SA-400 51⁄4-inch
floppy disk (1976) held 109,375 bytes unformatted,[12] and was advertised as "110 Kbyte", using the 1000 convention.[13] Likewise, the 8-inch
DEC RX01 floppy (1975) held 256,256 bytes formatted, and was advertised as "256k".[14] On the other hand, the
Tandon 51⁄4-inch
DD floppy format (1978) held 368,640 (which is 360×1024) bytes, but was advertised as "360 KB", following the 1024 convention.
On modern systems, all versions of
Microsoft Windows including the newest (as of 2019[update])
Windows 10 divide by 1024 and represent a 65,536-byte file as "64 KB".[9] Conversely,
Mac OS X Snow Leopard and newer represent this as 66 kB, rounding to the nearest 1000 bytes.[15] File sizes are reported with decimal prefixes.[16]
^
abcInternational Standard
IEC 80000-13 Quantities and Units – Part 13: Information science and technology, International Electrotechnical Commission (2008).
^Le Système international d’unités [The International System of Units] (PDF) (in French and English) (9th ed.), International Bureau of Weights and Measures, 2019, p. 143,
ISBN978-92-822-2272-0. "The SI prefixes refer strictly to powers of 10. They should not be used to indicate powers of 2 (for example, one kilobit represents 1000 bits and not 1024 bits). The names and symbols for prefixes to be used with powers of 2 are recommended as follows: kibi Ki 210 [...]"
^"Our Customer Terms"(PDF). Telstra. p. 7.
Archived(PDF) from the original on 10 April 2017. Retrieved 26 October 2016.
^"Vulgata Clementina". pater noster qui es in cælis sanctificetur nomen tuum adveniat regnum tuum fiat voluntas tua sicut in cælo et in terra panem nostrum supersubstantialem da nobis hodie et dimitte nobis debita nostra sicut et nos dimittimus debitoribus nostris et ne nos inducas in tentationem sed libera nos a malo