Template:Quantities of bytes

A kilobyte (derived from the SI prefix kilo-) is a unit of information or computer storage equal to either 1024 or 1000 bytes. It is commonly abbreviated KB, or informally K.

The term "kilobyte" was first loosely used for a value of 1024 bytes (210), because 210 is roughly one thousand and powers of two are convenient for use with binary digital computers. Unfortunately this abuse of the SI prefix got carried away from the slang of computer professionals into the mainstream lexicon by the marketing people, creating a lot of semantics problems.

Some suggested that the prefix K should be use to distinguish this quantity from the SI prefix k. However, the K prefix was never formally mandated and it is not used consistently. When larger units were needed for millions of bytes or more, the subtle upper-case / lower-case distinction between the SI prefix and this special use in computing, was not available (SI already uses the prefixes m and M to mean "thousandth" and "million" respectively). Higher-order SI prefixes are therefore used with either decimal (powers of 1000) or binary (powers of 1024) values, depending on context. See binary prefix for more details.

  • 1024 bytes (210): This definition is always used to express memory capacity, and other quantities which are based on powers of two. Most software also uses it to express storage capacity. However, this definition is explicitly forbidden by the SI standard. This quantity is expressed unambiguously as a kibibyte (KiB), and that name is recommended by most standards organizations. Although it is only seldomly seen in practice, it is starting to be adopted by software in which precision is important, such as BitTorrent or the Linux kernel.
  • 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. However, the overwhelming popularity of the 1024 definition means that anyone using "kilobyte" to mean 1000 is likely to cause confusion if any precision is needed. Still, it is convenient to use 1000 when deriving kilobyte measures from quantities which are often based on decimal multiples, such as clock speeds or bitrates.

For instance, a 512 kbit/s DSL line has a capacity of exactly 512000 bits/s. Dividing by 8, this is 64000 bytes/s, which is 62.5 KiB. However, the unpredictable levels of transmission overhead and error rate mean that the 2% error in referring to this as "64K" is fairly insignificant.

Compare with kilobit, which is frequently used with either a decimal or binary meaning.

See also

de:Kilobyte fr:Kilo-octet it:Kilobyte nl:Kilobyte sk:Kilobajt th:กิโลไบต์


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