Prefixes for bit and byte
Decimal Value SI 10001 k kilo- 10002 M mega- 10003 G giga- 10004 T tera- 10005 P peta- 10006 E exa- 10007 Z zetta- 10008 Y yotta-	Binary Value FOLDOC IEC 10241 K kilo- Ki kibi- 10242 M mega- Mi mebi- 10243 G giga- Gi gibi- 10244 T tera- Ti tebi- 10245 P peta- Pi pebi- 10246 E exa- Ei exbi- 10247 Z zetta- Zi zebi- 10248 Y yotta- Yi yobi-

A kilobyte (derived from the SI prefix kilo-, meaning 1,000) is a unit of information or computer storage equal to either 1,000 bytes or 1,024 bytes (2¹⁰), depending on context.

It can be abbreviated a number of ways: kB, K and Kbyte.

Ambiguity

Main article: Binary prefix

The exact number of bytes in a kilobyte has traditionally been ambiguous. Locations in electronic memory circuits are identified by binary numbers, which means that the number of addressable locations naturally becomes a power of 2, and memory sizes are therefore not integer multiples (or fractions) of 1000. However, as 2¹⁰ = 1024 ≈ 1000, the established 'k' (for kilo) was early on employed as a convenient "approximate" prefix for memory capacities in multiples of 1024. On the other hand, for products where (some) capacity factors were not equally bound to powers of two, such as magnetic disks (sector and track numbers) and networking equipment (bit rates), strict decimal-based units were used.

Some have suggested that the capitalized prefix K should be used to distinguish this quantity from the SI prefix k, but this has never been formally mandated. Further, it is not extensible to higher-order prefixes, as SI already uses the prefixes m and M to mean "milli-" and "mega-" respectively. There are also proposals to capitalize all greater-than-unity prefixes (D, H, K, M, G, ...), which would conflict with this. See SI prefix.

These prefixes can therefore be used with either decimal (powers of 1000) or binary (powers of 1024) values, depending on context:

• 1024 bytes (2¹⁰): This definition is used when expressing quantities which are based on powers of two, such as memory chip capacities. Most software also uses it to express storage capacity. This definition has been expressly forbidden by the SI standard^[1] and other standards organisations. To indicate a quantity of 1024 bytes, the term kibibyte (KiB) has been recommended instead.^[2][3] This term is starting to be adopted by some software, such as a few BitTorrent clients and the Linux kernel.^[4]

• 1000 bytes (10³): This definition is consistent with the SI prefix, and is recommended for all uses by international standards organizations such as IEC, IEEE, and ISO, with the abbreviation "kB". The overwhelming popularity of the 1024 definition for memory and file sizes means that anyone using "kilobyte" to mean 1000 in these situations is likely to cause confusion. However, it is common to use 1000 when deriving kilobyte measures from quantities which are not based on powers of two, such as bitrates.

Kilobyte (abbreviated as kB) is not to be confused with the term kilobit (abbreviated as kb).

See also

• Orders of magnitude (data)
• Megabyte

References

1. ^ [1] section 3.1, marginal note
2. ^ Prefixes for Binary Multiples — The NIST Reference on Constants, Units, and Uncertainty
3. ^ IEEE Std 260.1-2004, IEEE Standard Letter Symbols for Units of Measurement (SI Units, Customary Inch-Pound Units, and Certain Other Units), Revision of IEEE Std 260.1-1993 (IEEE, New York, 2004)
4. ^ Post on Linux Kernel Mailing List announcing the change.