Kilobyte

Kilobyte

Prefixes for bit and byte multiples

Decimal

Value SI

1000 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 IEC JEDEC

1024 Ki kibi- K kilo-

1024² Mi mebi- M mega-

1024³ Gi gibi- G giga-

1024⁴ Ti tebi-

1024⁵ Pi pebi-

1024⁶ Ei exbi-

1024⁷ Zi zebi-

1024⁸ Yi yobi-

A kilobyte (derived from the SI prefix kilo-, meaning 1,000) is a unit of digital information storage equal to either 1,024 bytes (2¹⁰) or 1,000 bytes (10³), depending on context.

It is abbreviated in a number of ways: KB, 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 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 are therefore used with either decimal (powers of 1000) or binary (powers of 1024) values, depending on context:

1024 bytes (2¹⁰): This unit is used when expressing quantities which are based on powers of two, such as memory chip capacities. Most software also expresses storage capacity in units of 1024 bytes. Although the use of kilobyte for this unit is common, this usage has been expressly forbidden by the SI standard¹ and other standards organisations. To indicate a quantity of 1024 bytes, the term kibibyte (KiB) has been recommended instead.²³

1000 bytes (10³): This definition is recommended for all uses by international standards organizations such as IEC, IEEE, and ISO,³ with the abbreviation "kB". This unit is common for quantities which are not based on powers of two, such as bitrates. This term is starting to be adopted by some software, such as the Linux kernel.⁴

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

References

^ [1] section 3.1, marginal note
^ Prefixes for Binary Multiples — The NIST Reference on Constants, Units, and Uncertainty
^ ^a ^b 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)
^ Post on Linux Kernel Mailing List announcing the change.

v • d • e Units of Information

Base units	Bit · Byte

Traditional units	Kilobyte · Megabyte · Gigabyte · Terabyte · Petabyte · Exabyte · Zettabyte · Yottabyte

IEC standard units	Kibibyte · Mebibyte · Gibibyte · Tebibyte · Pebibyte · Exbibyte · Zebibyte · Yobibyte

Ambiguity

See also

References