prefixes for binary multiples

The common practice of using the SI decimal multiple and submultiple prefixes with binary quantities, as in the term “kilobyte”, has lead to some confusion (see the discussion of megabyte in byte for examples).

The people in charge of maintaining SI were distressed by what they see as a misuse of the SI prefixes. The matter was discussed at the 12th meeting (1996) of the Consultative Committee for Units¹, and the next (7th) edition of the SI brochure explicitly condemned the practice. The CIPM adopted a resolution² in 1996 urging the International Electrotechnical Commission to devise another set of prefixes specifically for binary quantities, and the IEC published such a standard³ in 1999.

Factor Name Symbol Origin Derived from
2¹⁰ kibi- Ki kilobinary (2¹⁰)¹ kilo- (10³)¹
2²⁰ mebi- Mi megabinary (2¹⁰)² mega- (10³)²
2³⁰ gibi- Gi gigabinary (2¹⁰)³ giga- (10³)³
2⁴⁰ tebi- Ti terabinary (2¹⁰)⁴ tera- (10³)⁴
2⁵⁰ pebi- Pi petabinary (2¹⁰)⁵ peta- (10³)⁵
2⁶⁰ exbi- Ei exabinary (2¹⁰)⁶ exa- (10³)⁶

In English, the syllable “bi” in the prefixes is to be pronounced “bee,” like the insect. So “kibibyte” is pronounced “key-bee-bite”.

Applying this system of prefixes to the byte, for example, one gets:

1 kibibyte = 1024 bytes
1 kilobyte = 1000 bytes
1 mebibyte = 1,048,576 bytes
1 megabyte = 1,000,000 bytes
1 gibibyte = 1,073,741,824 bytes
1 gigabyte = 1,000,000,000 bytes
1 tebibyte = 1,099,511,627,776 bytes
1 terabyte = 1,000,000,000,000 bytes
1 pebibyte = 1,125,899,906,842,624 bytes
1 petabyte = 1,000,000,000,000,000 bytes
1 exbibyte = 1,152,921,504,606,846,976 bytes
1 exabyte = 1,000,000,000,000,000,000 bytes

1. BIPM Comité Consultatif des Unités, 1996, 12.

2. Recommendation 2 (CI-1996)
BIPM Proc.-Verb. Comité International des Poids et Mesures, volume 64, page 146. (1996)

3. International Standard IEC 60027-2, Letter symbols to be used in electrical technology –
Part 2: Telecommunications and Electronics, Amendment 2.

Geneva: International Electrotechnical Commission, 1999.

resources

A. J. Thor.
Prefixes for binary multiples. [letter to the editor]
Metrologia, volume 37, page 81 (2000).

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