v • d • e Quantities of bytes
SI prefixes (decimal)			IEC prefixes (binary)
	Binary usage (often with KB for kB)
Value	Name		Value	Name
10001 = 103	kilobyte	(kB)	10241 = 210 = 1.024·103	kibibyte	(KiB)
10002 = 106	megabyte	(MB)	10242 = 220 ≈ 1.049·106	mebibyte	(MiB)
10003 = 109	gigabyte	(GB)	10243 = 230 ≈ 1.074·109	gibibyte	(GiB)
10004 = 1012	terabyte	(TB)	10244 = 240 ≈ 1.100·1012	tebibyte	(TiB)
10005 = 1015	petabyte	(PB)	10245 = 250 ≈ 1.126·1015	pebibyte	(PiB)
10006 = 1018	exabyte	(EB)	10246 = 260 ≈ 1.153·1018	exbibyte	(EiB)
10007 = 1021	zettabyte	(ZB)	10247 = 270 ≈ 1.181·1021	zebibyte	(ZiB)
10008 = 1024	yottabyte	(YB)	10248 = 280 ≈ 1.209·1024	yobibyte	(YiB)

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: K, KB, Kbyte and kB.

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, and although this has never been formally mandated, it is commonly practiced (even though K is already used as the SI symbol for kelvins). However, it is not extensible to the higher-order prefixes, as SI already uses the prefixes m and M to mean "thousandth" and "million" 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 always used to express memory chip capacity, and other quantities which are based on powers of two. Most software also uses it to express storage capacity. This definition has been expressly forbidden by the SI standard ( section 3.1, marginal note), and, since 1998, most standards organizations instead recommend the term kibibyte (KiB). Although the word "kibibyte" is seldom seen in practice, it is starting to be adopted by some software, such as BitTorrent or the Linux kernel. Yet, according to Google test, its usage is less than 0.3%.

• 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 to kB with an upper case b) is not to be confused with the term kilobit (abbreviated to kb with a lower case b).