Kibi, mebi, gibi, tebi, pebi, and all that

Kibi, mebi, gibi, tebi, pebi, and exbi are binary prefix multipliers that, in 1998, were approved as a standard by the International Electrotechnical Commission (IEC) in an effort to eliminate the confusion that sometimes occurs between decimal (power-of-10) and binary (power-of-2) numeration terms.

Also see Kilo, mega, giga, tera, peta, and all that.

Kibi, mebi, gibi, tebi, pebi, and exbi are binary prefix multipliers that, in 1998, were approved as a standard by the International Electrotechnical Commission (IEC) in an effort to eliminate the confusion that sometimes occurs between decimal (power-of-10) and binary (power-of-2) numeration terms.

At present, the prefix multipliers kilo- (k or K), mega- (M), giga- (G), tera- (T), peta- (P), and exa- (E) are ambiguous. In most of the physical sciences, and when describing quantities of objects generally, these multipliers refer to powers of 10. However, when used to define data quantity in terms of bytes, they refer to powers of 2. The following table denotes the most often-used prefixes and their meanings.

 

Prefix Symbol(s) Power of 10 Power of 2
kilo- k or K ** 103 210
mega- M 106 220
giga- G 109 230
tera- T 1012 240
peta- P 1015 250
exa- E 1018 * 260
* Not generally used to express data speed
** k = 103 and K = 210

The power-of-10 multipliers and the power-of-2 multipliers for a given word prefix are almost, but not quite, the same. For example, the power-of-10 definition of kilo- (k) refers to 1,000, while the power-of-2 definition (K) refers to 1,024. As if this is not confusing enough, when referring to a data speed of one kilobit per second (1 kbps), analysts generally mean 1,000 bits per second (103 bps), but when talking about one kilobyte (1 KB) of data storage, they usually mean 1,024 bytes (210 B). This prevailing confusion could be eliminated (some computer scientists believe) by adopting special prefixes referring to the binary quantities. The proposed scheme is as follows.

 

Full technical
name
Proposed
prefix
Proposed
symbol
Numeric
multiplier
kilobinary kibi- Ki 210
megabinary mebi- Mi 220
gigabinary gibi- Gi 230
terabinary tebi- Ti 240
petabinary pebi- Pi 250
exabinary exbi- Ei 260

In scenarios such as the one mentioned above, if the new binary prefixes are used, it should be easy to know whether an engineer is talking or writing about the decimal or binary multiplier. We will know that one kilobit per second (1 kbps) means 1,000 bps, and one kibibyte (1 KiB) means 1,024 bytes, for example.

As of this writing, the binary prefix multipliers have not yet come into general use.

Pronunciation: Based on a suggestion from NIST, "the first syllable of the name of the binary-multiple prefix should be pronounced in the same way as the first syllable of the name of the corresponding International Standard (SI) prefix, and the second syllable should be pronounced as 'bee.'" Thus, "kibi" would be pronounced "KIH-bee"; "mebi" would be "MEH-bee", and so forth.

This was first published in April 2008

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