IEC prefixes and symbols for binary multiples

At the dawn of computing and electronics, computer professionals have noticed that 210 is almost equal to 1000, and started loosely using SI prefix kilo to mean 1024. As time passed, and as computers were introduced into mainstream life, and as the amounts of data people were dealing with grew dramatically, a great deal of confusion arose around this. Prefixes kilo and mega became to mean different things in different contexts. For example when describing computer memory, manufacturers usually use “megabyte” to mean 220 = 1 048 576 bytes, while manufacturers of storage devices usually use the term to mean 106 = 1 000 000 bytes. These two different meanings of the term can be found in many different areas of computing, and as if that is not enough confusion, a third “megabyte” of 1 024 000 bytes is the term used to format 90 mm (3½ inch), “1.44 MB” diskettes.

As the numbers increase, so does the gap between the actual base‐two number and its SI prefix approximation:

Differences between SI prefix approximations and actual base‐two numbers
SI Prefix SI Value Base‐two Value Difference
kilo 1000 1024 2%
mega 1000000 1048576 5%
giga 1000000000 1073741824 7%
tera 1000000000000 1099511627776 10%
peta 1000000000000000 1125899906842624 13%
exa 1000000000000000000 1152921504606846976 15%
zetta 1000000000000000000000 1180591620717411303424 18%
yotta 1000000000000000000000000 1208925819614629174706176 21%

Faced with this reality, in January 1997 the IEEE Standards Board has 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.

In 1998 the 7th edition of the SI Brochure was published, which included a marginal note, explicitly stating that SI prefixes should not be used to indicate powers of 2.

In December 1998 the International Electrotechnical Commission (IEC), the international organization for worldwide standardization in electrotechnology, approved as an IEC International Standard the names and symbols for prefixes for binary multiples for use in the fields of data processing and data transmission. The prefixes shall be used to indicate multiplication by 210n, where n = 1,2,3,4,5, or 6. The prefixes are as follows:

IEC prefixes and symbols for binary multiples
Number Factor Name Symbol Origin SI Derivation
1024 210 kibi Ki kilobinary (210)1 kilo: (103)1
1048576 220 mebi Mi megabinary (210)2 mega: (103)2
1073741824 230 gibi Gi gigabinary (210)3 giga: (103)3
1099511627776 240 tebi Ti terabinary (210)4 tera: (103)4
1125899906842624 250 pebi Pi petabinary (210)5 peta: (103)5
1152921504606846976 260 exbi Ei exabinary (210)6 exa: (103)6
1180591620717411303424 270 zebi Zi zettabinary (210)7 zetta: (103)7
1208925819614629174706176 280 yobi Yi yottabinary (210)8 yotta: (103)8

Thus, a 90 mm (3½ inch) floppy disk should be described as having a capacity of 1.41 MiB (equivalent to 1.47 MB), not “1.44 MB” as labelled. 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”.

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.)

These prefixes for binary multiples, which were developed by the IEC Technical Committee (TC) 25, Quantities and Units, and Their Letter Symbols, with the strong support of CIPM, IEEE and ISO, were first adopted by the IEC as Amendment 2 to IEC International Standard IEC 60027‐2: Letter symbols to be used in electrical technology — Part 2: Telecommunications and electronics. The full content of Amendment 2, which has a publication date of 1999‐01, is reflected in the table above and the suggestion regarding pronunciation. Subsequently the contents of this Amendment were incorporated in the second edition of IEC 60027‐2, which has a publication date of 2000‐11 (the first edition was published in 1972). The complete citation for this revised standard is IEC 60027‐2, Second edition, 2000‐11, Letter symbols to be used in electrical technology — Part 2: Telecommunications and electronics.

In 2001 the American NIST has published Special Publication 330, 2001 Edition (US version of the 7th edition of the SI Brochure), in which editor included an extra marginal note with a description, stating that IEC standard prefixes should be used in the field of information technology to avoid the incorrect usage of the SI prefixes.

After a much heated discussion in December 2001 on linux‐kernel mailing list, the binary prefixes have been accepted by the key Linux developers, and are now extensively gaining ground across Open Source UNIX applications.

On December 11 2002 the IEEE‐SA has approved proposal P1541 as a Trial‐Use Standard for the duration of 2 years. IEEE Standard No.: 1541‐2002, Name: IEEE Standard for Prefixes for Binary Multiples, ISBN: 0‐73813385‐X. It can be purchased at IEEE Online Store. The standard was prepared by Standards Coordinating Committee (SCC) 14, Quantities, Units, and Letter Symbols.

BSR at ANSI has also accepted BSR/IEEE 1541‐200x, Trial‐Use Standard for Prefixes for Binary Multiples (trial use standard) for a trial period: 1 December, 2002 through 30 June, 2004.

In April 2003 CIPM has held a CCU meeting at BIPM, at which it has been agreed to also include a marginal note discussing the binary multiples in the upcoming 8th edition of the SI Brochure.

On September 18 2003 the Reuters has reported that Apple, Dell, Gateway, Hewlett‐Packard, IBM, Sharp, Sony and Toshiba have been sued in a class‐action suit in Los Angeles Superior Court for “deceiving” the true capacity of their hard drives. This of course was due to ambiguity of “GB” when used by software and hardware vendors. This precedent might prompt Apple to adapt binary prefixes in its Mac OS, as well as other companies to put pressure on Microsoft to adapt them in its Windows operating systems.

On November 10 2003 the W3C has published a Working Group Note Units in MathML in which it showed how IEC prefixes should be incorporated into mathematical markup.

On Feburary 20 2004 another lawsuit emerged, this time against flash drive manufacturers

On March 19 2005 the IEEE‐SA Standards Board Standards Review Committee has elevated status of the 1541‐2002 standard from trial‐use to full‐use and scheduled the standard for a maintenance action in 2007.

The ANSI has followed suit on April 29 2005 accepting BSR/IEEE 1541‐200x, Prefixes for Binary Multiples as a new standard.

On August 11 2005 the IEC has published the third edition of IEC 60027‐2 and made an announcement about inclusion of prefixes zebi and yobi in the standard.

In May 2006 the BIPM has published the 8th edition of SI brochure with an extended marginal note that describes IEC 60027‐2 standard prefixes and advocates their use in the field of information technology to avoid incorrect usage of SI prefixes.

In June 2006 Western Digital Corporation has settled the class action lawsuit entitled Orin Safier v. Western Digital Corporation et al., Case No. 05-03353 BZ before Judge Bernard Zimmerman in the United States District Court for the Northern District of California.

List of software that uses IEC prefixes

Last Modified: 1 Jul 2006

See also: SI prefixes for decimal multiples.