Binary prefix

Source: NIST


In computing, binary prefixes frequently are used to quantify large numbers where powers of two are more useful than powers of ten. Each successive prefix is multiplied by 1024 (210) rather than the 1000 (103) used by the SI prefix system.

In December 1998 the International Electrotechnical Commission (IEC), the leading international organization for worldwide standardization in electrotechnology, approved as an IEC International Standard names and symbols for prefixes for binary multiples for use in the fields of data processing and data transmission. The prefixes are as follows:

Factor Name Symbol Origin SI derivation
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
270 zebi Zi zettabinary (210)7 zetta (103)7
280 yobi Yi yottabinary (210)8 yotta (103)8

Examples and comparisons with SI prefixes from:

one kibibit 1 Kibit = 210 bit = 1024 bit
one kilobit 1 kbit = 103 bit = 1000 bit
one mebibyte 1 MiB = 220 B = 1,048,576 B
one megabyte 1 MB = 106 B = 1,000,000 B
one gibibyte 1 GiB = 230 B = 1,073,741,824 B
one gigabyte 1 GB = 109 B = 1,000,000,000 B

It is suggested that in English, 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 SI prefix, and that the second syllable should be pronounced as "bee."

It is important to recognize that the new prefixes for binary multiples are not part of the International System of Units (SI), the modern metric system. However, for ease of understanding and recall, they were derived from the SI prefixes for positive powers of ten. As can be seen from the above table, the name of each new prefix is derived from the name of the corresponding SI prefix by retaining the first two letters of the name of the SI prefix and adding the letters "bi," which recalls the word "binary." Similarly, the symbol of each new prefix is derived from the symbol of the corresponding SI prefix by adding the letter "i," which again recalls the word "binary." (For consistency with the other prefixes for binary multiples, the symbol Ki is used for 210 rather than ki.)


Once upon a time, computer professionals noticed that 210 was very nearly equal to 1000 and started using the SI prefix "kilo" to mean 1024. That worked well enough for a decade or two because everybody who talked kilobytes knew that the term implied 1024 bytes. But, almost overnight a much more numerous "everybody" bought computers, and the trade computer professionals needed to talk to physicists and engineers and even to ordinary people, most of whom know that a kilometer is 1000 meters and a kilogram is 1000 grams.

Then data storage for gigabytes, and even terabytes, became practical, and the storage devices were not constructed on binary trees, which meant that, for many practical purposes, binary arithmetic was less convenient than decimal arithmetic. The result is that today "everybody" does not "know" what a megabyte is. When discussing computer memory, most manufacturers use megabyte to mean 220 = 1 048 576 bytes, but the manufacturers of computer storage devices usually use the term to mean 1 000 000 bytes. Some designers of local area networks have used megabit per second to mean 1,048,576 bit/s, but all telecommunications engineers use it to mean 106 bit/s. And if two definitions of the megabyte are not enough, a third megabyte of 1,024,000 bytes is the megabyte used to format the familiar 90 mm (3 1/2 inch), "1.44 MB" diskette. The confusion is real, as is the potential for incompatibility in standards and in implemented systems.

Faced with this reality, the Institute of Electrical and Electronics Engineers (IEEE) Standards Board decided that IEEE standards will use the conventional, internationally adopted, definitions of the SI prefixes. "Mega" will mean 1,000,000, except that the base-two definition may be used (if such usage is explicitly pointed out on a case-by-case basis) until such time that prefixes for binary multiples are adopted by an appropriate standards body.

Disclaimer: This article is taken wholly from, or contains information that was originally published by, the National Institue of Standards and Technology. Topic editors and authors for the Encyclopedia of Earth may have edited its content or added new information. The use of information from the National Institue of Standards and Technology should not be construed as support for or endorsement by that organization for any new information added by EoE personnel, or for any editing of the original content.



(2008). Binary prefix. Retrieved from


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