Binary prefix

This article is about powers-of-two prefixes for measurement units like bit and byte. For notations for the radix of a numeral, see Integer literal § Affixes.

"Gibi" redirects here. For other uses, see Gibi (disambiguation).

Prefixes for decimal and binary multiples

Decimal			Binary
Value	SI		Value	IEC		JEDEC
1000	k	kilo	1024	Ki	kibi	K	kilo
10002	M	mega	10242	Mi	mebi	M	mega
10003	G	giga	10243	Gi	gibi	G	giga
10004	T	tera	10244	Ti	tebi	T	tera
10005	P	peta	10245	Pi	pebi	—
10006	E	exa	10246	Ei	exbi	—
10007	Z	zetta	10247	Zi	zebi	—
10008	Y	yotta	10248	Yi	yobi	—
10009	R	ronna	10249	Ri	robi	—
100010	Q	quetta	102410	Qi	quebi	—

A binary prefix is a unit prefix that indicates a multiple of a unit of measurement by an integer power of two. The most commonly used binary prefixes are kibi (symbol Ki, meaning 2¹⁰ = 1024), mebi (Mi, 2²⁰ = 1048576), and gibi (Gi, 2³⁰ = 1073741824). They are most often used in information technology as multipliers of bit and byte, when expressing the capacity of storage devices or the size of computer files.

The binary prefixes "kibi", "mebi", etc. were defined in 1999 by the International Electrotechnical Commission (IEC), in the IEC 60027-2 standard (Amendment 2). They were meant to replace the metric (SI) decimal power prefixes, such as "kilo" (k, 10³ = 1000), "mega" (M, 10⁶ = 1000000) and "giga" (G, 10⁹ = 1000000000), that were commonly used in the computer industry to indicate the nearest powers of two. For example, a memory module whose capacity was specified by the manufacturer as "2 megabytes" or "2 MB" would hold 2 × 2²⁰ = 2097152 bytes, instead of 2 × 10⁶ = 2000000.

On the other hand, a hard disk whose capacity is specified by the manufacturer as "10 gigabytes" or "10 GB", holds 10 × 10⁹ = 10000000000 bytes, or a little more than that, but less than 10 × 2³⁰ = 10737418240 and a file whose size is listed as "2.3 GB" may have a size closer to 2.3 × 2³⁰ ≈ 2470000000 or to 2.3 × 10⁹ = 2300000000, depending on the program or operating system providing that measurement. This kind of ambiguity is often confusing to computer system users and has resulted in lawsuits. The IEC 60027-2 binary prefixes have been incorporated in the ISO/IEC 80000 standard and are supported by other standards bodies, including the International Bureau of Weights and Measures (BIPM), which defines the SI system, the US National Institute of Standards and Technology (NIST), and the European Union.

Prior to the 1999 IEC standard, some industry organizations, such as the Joint Electron Device Engineering Council (JEDEC), noted the common use of the terms kilobyte, megabyte, and gigabyte, and the corresponding symbols KB, MB, and GB in the binary sense, for use in storage capacity measurements. However, other computer industry sectors (such as magnetic storage) continued using those same terms and symbols with the decimal meaning. Since then, the major standards organizations have expressly disapproved the use of SI prefixes to denote binary multiples, and recommended or mandated the use of the IEC prefixes for that purpose, but the use of SI prefixes in this sense has persisted in some fields.