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{{Short description|Indicates multiples or fractions of a unit}}
A '''unit prefix''' is a specifier or mnemonic that is added to the beginning of a [[unit of measurement]] to indicate multiples or fractions of the units. Units of various [[order of magnitude|sizes]] are commonly formed by the use of such [[prefix]]es. The [[Metric prefix|prefixes of the metric system]], such as ''[[kilo-|kilo]]'' and ''[[milli-|milli]]'', represent multiplication by positive or negative [[exponentiation|powers]] of ten.  In information technology it is common to use [[binary prefixes]], which are based on [[power of 2|powers of two]].  Historically, many prefixes have been used or proposed by various sources, but only a narrow set has been recognised by standards organisations.

== Metric prefixes ==
{{main|Metric prefix}}
{{common metric prefixes}}
The prefixes of the [[metric system]] precede a basic unit of measure to indicate a [[decimal|decadic]] [[multiple (mathematics)|multiple]] and [[fraction (mathematics)|fraction]] of a unit. Each prefix has a unique symbol that is added to the beginning of the unit symbol. Some of the prefixes date back to the introduction of the metric system in the 1790s, but new prefixes have been added, and some have been revised. The [[International Bureau of Weights and Measures]] has standardised twenty metric prefixes in resolutions dating from 1960 to 1991 for use with the [[International System of Units]] (SI).<ref>{{cite web |url=http://www.bipm.org/en/si/prefixes.html |title=Four Resolutions |publisher=Bipm.org |access-date=2012-03-01}}</ref> In addition to those listed in the everyday-use table, the SI includes standardised prefixes for 10<sup>15</sup> ([[Peta (SI prefix)|peta]]), 10<sup>18</sup> ([[Exa (SI prefix)|exa]]), 10<sup>21</sup> ([[zetta]]), 10<sup>24</sup> ([[yotta]]), 10<sup>27</sup> ([[ronna-|ronna]]), and 10<sup>30</sup> ([[quetta-|quetta]]); and for 10<sup>−15</sup> ([[femto]]), 10<sup>−18</sup> ([[atto-|atto]]), 10<sup>−21</sup> (zepto), 10<sup>−24</sup> ([[yocto]]), 10<sup>−27</sup> ([[ronto-|ronto]]), and 10<sup>−30</sup> ([[quecto]]).

[[File:Myriameterstein36RüdesheimRhein.JPG|250px|left|thumb|[[Distance marker]] on the [[Rhine]]: 36 (XXXVI) myriametres from [[Basel]]]]

Although formerly in use, the SI disallows combining prefixes; the *''microkilogram'' or *''centimillimetre'', for example, are not permitted.  Prefixes corresponding to powers of one thousand are usually preferred, however, units such as the hectopascal, centimetre, and centilitre, are widely used; outside the SI, the units [[hectare]], [[decibel]] are also common. The unit prefixes are always considered to be part of the unit, so that, e.g., in [[exponentiation]], 1&nbsp;km<sup>2</sup> means one square kilometre, not one thousand square metres, and 1&nbsp;cm<sup>3</sup> means one cubic centimetre, not one hundredth of a cubic metre.

In general, prefixes are used with any metric unit, but may also be used with non-metric units. Some combinations, however, are more common than others. The choice of prefixes for a given unit has often arisen by convenience of use and historical developments. Unit prefixes that are much larger or smaller than encountered in practice are seldom used, albeit valid combinations. In most contexts only a few, the most common, combinations are established. For example, prefixes for multiples greater than one thousand are rarely applied to the gram or metre.

Some prefixes used in older versions of the metric system are no longer used. The prefixes ''[[myria (metric prefix)|myria-]]'',<ref>{{cite web |url=http://lamar.colostate.edu/~hillger/laws/metric-act-bill.html |title=H.R. 596, An Act to authorise the use of the metric system of weights and measures | date=1866-05-13 |url-status=dead |archiveurl=https://web.archive.org/web/20130114025147/http://lamar.colostate.edu/~hillger/laws/metric-act-bill.html |archivedate=2013-01-14 }}</ref><ref name="Brewster_1830">{{cite book |title=The Edinburgh Encyclopædia |author-first=David |author-last=Brewster |volume=12 |date=1830 |location=Edinburgh, UK |publisher=William Blackwood, John Waugh, John Murray, Baldwin & Cradock, J. M. Richardson |page=494 |url=https://books.google.com/books?id=0bIkTUZAbxcC |access-date=2015-10-09}}</ref><ref name="Brewster_1832">{{cite book |title=The Edinburgh Encyclopaedia |author-first=David |author-last=Brewster |volume=12 |edition=1st American |date=1832 |publisher=Joseph and Edward Parker<!-- |printer=William Brows --> |url=https://books.google.com/books?id=17RGAQAAIAAJ&pg=PA572 |access-date=2015-10-09}}</ref> (from the Greek [[myriad|μύριοι]], ''mýrioi''), ''[[double (metric prefix)|double-]]'' and ''[[demi (metric prefix)|demi-]]'', denoting factors of {{val|10000}}, 2 and {{frac|2}} respectively,<ref name="fr">{{cite web |title=La Loi Du 18 Germinal An 3: Décision de tracer le mètre, unité fondamentale, sur une règle de platine. Nomenclature des " mesures républicaines ". Reprise de la triangulation. |language=fr |trans-title=The Law of 18 [[Germinal (month)|Germinal]], Year 3: Decision to draw the fundamental unit metre on a platinum ruler. Nomenclature of "Republican measures". Resumption of the triangulation. |work=L'Histoire Du Mètre [The history of the metre] |publisher=histoire.du.metre.free.fr |url=http://histoire.du.metre.free.fr/fr/Pages/Sommaire/06.htm |access-date=2015-10-12 |url-status=live |archive-url=https://web.archive.org/web/20221126164814/http://histoire.du.metre.free.fr]/fr/Pages/Sommaire/06.htm |archive-date=2022-11-26 |quote=Art. 8. Dans les poids et mesures de capacité, chacune des mesures décimales de ces deux genres aura son double et sa moitié, afin de donner à la vente des divers objets toute la commodité que l'on peut désirer. Il y aura donc le double-litre et le demi-litre, le double-hectogramme et le demi-hectogramme, et ainsi des autres. |trans-quote=Art. 8. In the weights and measures of capacity, each of the decimal measures of these two kinds will have its double and its half, in order to give to the sale of the various articles all the convenience that one can desire. There will therefore be the double-litre and the half-litre, the double-hectogram and the half-hectogram, and so on.}}</ref> were parts of the original metric system adopted in France in 1795, but they were not retained when the SI prefixes were agreed internationally by the 11th [[Conférence générale des poids et mesures|CGPM conference]] in 1960. The prefix "''[[myrio-]]''" was an alternative spelling variant for "''myria-''", as proposed by [[Thomas Young (scientist)|Thomas Young]].
<ref name="Brewster_1830"/><ref name="Brewster_1832"/><ref name="Dingler_1823">{{cite book |title=Polytechnisches Journal |author-first=Johann Gottfried |author-last=Dingler |volume=11 |date=1823 |publisher=J.W. Gotta'schen Buchhandlung |language=German |location=Stuttgart, Germany |url=https://books.google.com/books?id=wF3zAAAAMAAJ&pg=PA500 |access-date=2015-10-09}}</ref><ref name="Shrivatav_1971">{{cite book |chapter-url=https://books.google.com/books?id=hqE8AAAAIAAJ&pg=PA785 |title=Gazetteer of India: Madhya Pradesh District Gazetteers - Indore |page=785 |chapter=Appendix B - XII Conversion Table |editor-first=P. N. |editor-last=Shrivatav |publisher=District Gazetteers Department, Madhya Pradesh, Bhopal |date=1971}}</ref>

== Binary prefixes ==
{| class="wikitable floatright"
|+[[Binary prefix]]es 
|-
! Prefix
! Symbol
! Power
|-
|yobi
| style="text-align: center;" |Yi
| style="text-align: center;" |2<sup>80</sup>
|-
|zebi
| style="text-align: center;" |Zi
| style="text-align: center;" |2<sup>70</sup>
|-
|exbi
| style="text-align: center;" |Ei
| style="text-align: center;" |2<sup>60</sup>
|-
|pebi
| style="text-align: center;" |Pi
| style="text-align: center;" |2<sup>50</sup>
|- 
|tebi
| style="text-align: center;" |Ti
| style="text-align: center;" |2<sup>40</sup>
|-
|gibi
| style="text-align: center;" |Gi
| style="text-align: center;" |2<sup>30</sup>
|-
|mebi
| style="text-align: center;" |Mi
| style="text-align: center;" |2<sup>20</sup>
|-
|kibi
| style="text-align: center;" |Ki
| style="text-align: center;" |2<sup>10</sup>
|- 
|}

A [[binary prefix]] indicates multiplication by a power of two. The tenth power of 2 (2<sup>10</sup>) has the value {{val|1024}}, which is close to {{val|1000}}. This has prompted the use of the metric prefixes ''kilo'', ''mega'', and ''giga'' to also denote the powers of 1024 which is common in information technology with the unit of digital information, the [[byte]].

Units of information are not covered in the International System of Units. Computer professionals have historically used the same spelling, pronunciation and symbols for the binary series in the description of [[computer memory]], although the symbol for ''kilo'' is often capitalised. For example, in citations of main memory or [[random-access memory|RAM]] capacity, ''kilobyte'', ''megabyte'' and ''gigabyte'' customarily mean {{val|1024}} (2<sup>10</sup>), {{val|1048576}} (2<sup>20</sup>) and {{val|1073741824}} (2<sup>30</sup>) bytes respectively.

In the specifications of [[hard disk drive]] capacities and network transmission [[bit rate]]s, decimal prefixes are used. For example, a 500-gigabyte hard drive holds 500 billion bytes, and a 100-megabit-per-second Ethernet connection transfers data at 100 million bits per second. The ambiguity has led to some confusion and even lawsuits from purchasers who were expecting 2<sup>20</sup> or 2<sup>30</sup> and considered themselves shortchanged by the seller. (see ''[[Orin Safier v. Western Digital Corporation]]'' and ''[[Cho v. Seagate Technology (US) Holdings, Inc.]]'').<ref>{{cite web |url=https://arstechnica.com/old/content/2006/06/7174.ars |title=Western Digital settles drive size lawsuit |author-first=Jeremy |author-last=Reimer |date=2006-06-30 |publisher=Ars Technica LLC |access-date=2012-02-18}}</ref><ref>[http://www.bit-tech.net/news/bits/2007/10/26/seagate_lawsuit_concludes_settlement_announced/1 Seagate lawsuit concludes, settlement announced], bit-tech.net</ref> To protect themselves, some sellers write out the full term as "{{val|1000000}}".

With the aim of avoiding ambiguity the [[International Electrotechnical Commission]] (IEC) adopted new binary prefixes in 1998 ([[ISO/IEC 80000|IEC 80000-13:2008]] formerly subclauses 3.8 and 3.9 of [[IEC 60027|IEC 60027-2:2005]]). Each binary prefix is formed from the first syllable of the decimal prefix with the similar value, and the syllable "bi". The symbols are the decimal symbol, always capitalised, followed by the letter "i".  According to these standards, ''kilo'', ''mega'', ''giga'', et seq. should only be used in the decimal sense, even when referring to data storage capacities: [[kilobyte]] and [[megabyte]] denote one thousand and one million bytes respectively (consistent with the metric system), while terms such as [[kibibyte]], [[mebibyte]] and [[gibibyte]], with symbols KiB, MiB and GiB, denote 2<sup>10</sup>, 2<sup>20</sup> and 2<sup>30</sup> bytes respectively.<ref>{{cite web |url=http://physics.nist.gov/cuu/Units/binary.html |title=International System of Units (SI): Prefixes for binary multiples |work=The NIST Reference on Constants, Units, and Uncertainty |publisher=[[National Institute of Science and Technology]] |access-date=2007-09-09}}</ref>

== Unofficial prefixes ==
Although some of the following unofficial prefixes appear repeated on the internet, none are in use.<ref>{{Cite web |url=http://mrob.com/pub/math/ln-notes1-3.html |access-date=2022-12-24 |url-status=live |title=Large Numbers – Notes (page 3) at MROB |archive-url=https://web.archive.org/web/20221221184811/http://www.mrob.com/pub/math/ln-notes1-3.html |archive-date=2022-12-21}}</ref>

A metric prefix ''[[myria-|myria]]'' (abbreviation "my"), for 10,000, was deprecated in 1960. Before the adoption of ronna and quetta for 10<sup>27</sup> and 10<sup>30</sup> and ronto and quecto for 10<sup>−27</sup> and 10<sup>−30</sup> in November 2022, many personal, and sometimes facetious, proposals for additional metric prefixes were formulated.<ref>{{cite web |title=Current and Deprecated Prefixes |editor-first=Gerard P. |editor-last=Michon |author=Calvin |date=2021 |orig-date=2020, 2000-11-05 |url=http://www.numericana.com/answer/units.htm#prefix |access-date=2022-12-24 |url-status=live |archive-url=https://web.archive.org/web/20221221184721/http://www.numericana.com/answer/units.htm#prefix |archive-date=2022-12-21}}</ref><ref>{{cite web |title=Funny prefixes & dubious proposals (updated yearly) |author-first=John |author-last=Foley |date=2002-05-27 |url=http://www.numericana.com/answer/humor.htm#prefixes |access-date=2022-12-24 |url-status=live |archive-url=https://web.archive.org/web/20221224130625/http://www.numericana.com/answer/humor.htm#prefixes |archive-date=2022-12-24}}</ref> The prefix ''bronto'', as used in the term "brontobyte", has been used to represent anything from 10<sup>15</sup> to 10<sup>27</sup> bytes, most often 10<sup>27</sup>.<ref name="Forbes">{{cite web |title=Extreme Big Data: Beyond Zettabytes And Yottabytes |author-first=Gerard P. |author-last=Michon |date=2013-10-09 |website=[[Forbes]] |url=https://www.forbes.com/sites/oracle/2013/10/09/extreme-big-data-beyond-zettabytes-and-yottabytes/ |access-date=2022-12-24 |url-status=live |archive-url=https://web.archive.org/web/20221224125425/https://www.forbes.com/sites/oracle/2013/10/09/extreme-big-data-beyond-zettabytes-and-yottabytes/?sh=4fa307777b2c |archive-date=2022-12-24}}</ref><ref>{{Cite web|url=http://news.bbc.co.uk/1/hi/magazine/3673262.stm<!-- https://web.archive.org/web/20220708000542/http://news.bbc.co.uk/2/hi/uk_news/magazine/3673262.stm -->|title=BBC article suggesting that a "brontobyte" is 10<sup>27</sup> bytes}}</ref><ref>{{Cite journal | url=http://www.sybase.com/content/1031253/EnterpriseUnwired.pdf#page=3 | journal=Sybase Magazine | issue=Q3 2004 | page=25| title=Unwired Enterprise Products | archive-url=https://web.archive.org/web/20141018093457/http://www.sybase.com/content/1031253/EnterpriseUnwired.pdf | archive-date=2014-10-18}}</ref><ref>{{cite web | url=http://www.pcsndreams.com/Pages/Articles/Megabytes.htm | title=Redirect }}</ref><ref>{{cite journal |journal=[[MacUser]] |date=1991-02-16 |volume=7 |page=362 |url=https://books.google.com/books?id=j31VAAAAMAAJ&q=brontobyte |quote=1 brontobyte (1,000,000,000 megabytes) |title=(undefined) |archive-url=https://web.archive.org/web/20221224131315/https://books.google.de/books?id=j31VAAAAMAAJ&q=brontobyte&redir_esc=y |archive-date=2022-12-24}}</ref> 
In 2010, an online petition sought to establish ''[[Hella (word)|hella-]]'' as the SI prefix for 10<sup>27</sup>, a movement that began on the campus of [[UC Davis]].<ref>{{cite magazine |url=http://www.time.com/time/health/article/0,8599,1970849,00.html |archive-url=https://web.archive.org/web/20100313005555/http://www.time.com/time/health/article/0,8599,1970849,00.html |url-status=dead |archive-date=March 13, 2010 |magazine=Time |title=Hellabytes? A Campaign to Turn Slang into Science |date=2010-03-10 |access-date=2010-05-20}}</ref><ref name='Telegraph 2 March 2010'>{{cite news |author-first=Matthew |author-last=Moore |title=Hella number: scientists call for new word for 1,000,000,000,000,000,000,000,000,000 |date=2010-03-02 |url=https://www.telegraph.co.uk/news/science/science-news/7352204/Hella-number-scientists-call-for-new-word-for-1000000000000000000000000000.html |work=The Telegraph |access-date=2019-01-04 |archive-url=https://web.archive.org/web/20221224131440/https://www.telegraph.co.uk/news/science/science-news/7352204/Hella-number-scientists-call-for-new-word-for-1000000000000000000000000000.html |archive-date=2022-12-24 |quote=More than 20,000 scientists, students and members of the public have signed an online petition backing the new quantity, which would be used for figures with 27 zeros after the first digit.}}</ref> The prefix, which has since appeared in the [[San Francisco Chronicle]], [[The Daily Telegraph|Daily Telegraph]], ''[[Wired (magazine)|Wired]]'' and some other scientific magazines, was recognised by [[Google]], in a [[Easter egg (media)|non-serious fashion]], in May 2010.<ref name='Wired 2010 June'>{{cite magazine |title=Jargon Watch |magazine=[[Wired (magazine)|Wired]] |date=June 2010 |volume=18 |issue=6 |quote=[…] a proposed metric prefix […] useful for describing mega-measurements like Earth's mass (6 Hellagrams). A Facebook petition garnered {{val|30000}} signatures}}</ref><ref name='Petition'>{{cite web |url=http://www.facebook.com/pages/The-Official-Petition-to-Establish-Hella-as-the-SI-Prefix-for-1027/277479937276?v=info |title=The Official Petition to Establish "Hella-" as the SI Prefix for 10^27 |access-date=2010-06-04 |publisher=[[Facebook]]}}</ref><ref name="Kim 24 May 2010">{{cite news |date=2010-05-24 |title=Google gets behind 'hella' campaign |url=http://www.sfgate.com/cgi-bin/blogs/techchron/detail?&entry_id=64240 |url-status=deviated |archive-url=https://web.archive.org/web/20100619034626/http://www.sfgate.com/cgi-bin/blogs/techchron/detail?&entry_id=64240 |archive-date=2010-06-19 |access-date=2010-06-04 |work=The San Francisco Chronicle |author-first=Ryan |author-last=Kim}}</ref> Ian Mills, president of the Consultative Committee on Units, considered the chances of official adoption to be remote.<ref>{{cite journal |url=http://articles.latimes.com/2010/jul/06/local/la-me-hella-20100706/2 |archive-url=https://web.archive.org/web/20100711004402/http://articles.latimes.com/2010/jul/06/local/la-me-hella-20100706/2 |url-status=dead |archive-date=July 11, 2010 |page=2 |title=Physics major has a name for a really big number |journal=[[Los Angeles Times]] |date=2010-06-06 |author-last=Chawkins |author-first=Steve}}</ref> The prefix ''geop'' and term "geopbyte" have been used in the [[information technology]] industry to refer to 10<sup>30</sup> bytes, following "brontobyte".<ref name="Forbes"/>

The ascending prefixes ''peta'' ({{val|1000}}<sup>5</sup>) and ''exa'' ({{val|1000}}<sup>6</sup>) are based on the Greek-derived [[numeric prefix]]es "penta" (5) and "hexa" (6). The largest prefixes ''zetta'' ({{val|1000}}<sup>7</sup>), and ''yotta'' ({{val|1000}}<sup>8</sup>) and, similarly, the descending prefixes ''zepto'' ({{val|1000}}<sup>&minus;7</sup>) and ''yocto'' ({{val|1000}}<sup>&minus;8</sup>) are derived from Latin<ref>[https://www.bipm.org/en/committees/cg/cgpm/19-1991 Proceedings of the 19th CGPM (1991)], page 80.</ref> "septem" (7) and " octo" (8) plus the initial letters "z" and "y". The initial letters "z" and "y" appear in the largest SI prefixes. They were changed because of previously proposed ascending ''hepto'' (Greek "hepta" (7)) was already in use as a numerical prefix (implying seven) and the letter "h" as both SI-accepted non-SI unit (hour) and prefix (''hecto'' 10<sup>2</sup>), the same applied to "s" from previously proposed descending ''septo'' (i.e. SI unit "s", seconds), while "o" for ''octo'' was problematic since a symbol "o" could be confused with zero.<ref group="nb">"The names ''zepto'' and ''zetta'' suggest the digit seven [''sept''] (seventh power of 10<sup>3</sup>) and the letter 'z' replaces the letter 's' to avoid the duplicate use of the letter 's' as a symbol. The names ''yocto'' and ''yotta'' are derived from ''octo'', which suggests the number eight (eighth power of 10<sup>3</sup>); the letter 'y' is added to avoid the use of the letter 'o' as a symbol because of the possible confusion with the digit zero." [https://www.bipm.org/en/committees/cg/cgpm/19-1991/resolution-4<!-- https://web.archive.org/web/20221224131711/https://www.bipm.org/en/committees/cg/cgpm/19-1991/resolution-4 --> Resolution 4 of the 19th CGPM (1991)]</ref>

Before the adoption of new prefixes in 2022, several personal proposals had been made for extending the series of prefixes, with ascending terms such as ''xenna'', ''weka'', ''vendeka'' (from Greek "ennea" (9), "deka" (10), "endeka" (11)) and descending terms such as ''xono'', ''weco'', ''vundo'' (from Latin "novem"/"nona" (9), "decem" (10), "undecim" (11)). Using Greek for ascending and Latin for descending would be consistent with established prefixes such as ''deca'', ''hecto'', ''kilo'' vs. ''deci'', ''centi'', ''milli''.<ref>[http://physics.info/system-international/ "International System"] in Glenn Elert, ''The Physics Hypertextbook''</ref>

In 2001, a few unofficial prefixes appeared on the Internet: ''hepa'' (10<sup>21</sup>), ''ento'' (10<sup>−21</sup>), ''otta'' (10<sup>24</sup>), ''fito'' (10<sup>−24</sup>), ''nea'' (10<sup>27</sup>), ''syto'' (10<sup>−27</sup>), ''dea'' (10<sup>30</sup>), ''tredo'' (10<sup>−30</sup>), ''una'' (10<sup>33</sup>) and ''revo'' (10<sup>−33</sup>). The Oxford professor [[Jeffrey K. Aronson]] has suggested extending beyond ''zetta/zepto'' and ''yotta/yocto'' with ''xenta/xenno'', ''wekta/weko'', ''vendeka/vendeko'', and ''udeka/udeko'', based on the idea that the "Z" and "Y" prefixes would continue backwards through the English alphabet. He goes on to list a large number of prefixes, starting with ''Xona, Weka, Vunda, Uda, Treda, Sorta'', ... Another proposal for ''xenta/xona'' is ''novetta'', from the Italian "nove" (or "nine"). In 1993, Morgan Burke proposed, as a joke, ''harpo'' for 10<sup>−27</sup>, ''groucho'' for 10<sup>−30</sup> (and therefore ''harpi'' for 10<sup>27</sup>, ''grouchi'' for 10<sup>30</sup>, ''zeppi'' for 10<sup>33</sup>, ''gummi'' for 10<sup>36</sup>, and ''chici'' for 10<sup>39</sup>), all of these being references to the comedy act, the [[Marx Brothers]].<ref>Gyllenbok, Jan; ''Encyclopaedia of Historical Metrology, Weights, and Measures'', volume 1, Science Networks Historical Studies 56, Birkhäuser/Springer International Publishing AG, 2010, ISBN 978-3-319-57596-4, [https://books.google.com/books?id=XnRVDwAAQBAJ&pg=PA204 page 204]</ref>

==Use for quantities of bits and bytes==
{{Bit and byte prefixes}}
Either metric and binary prefixes are used for representing quantities of bits and bytes.

== See also ==
* [[Order of magnitude]]
* [[SI base unit]]
* [[Indian numbering system]]

== Notes ==
{{reflist|group="nb"}}

== References ==
{{reflist|30em}}

== External links ==
* [http://www.stanford.edu/group/moerner/guacamole.html Moerner Lab Single-Molecule Research Page] (Jokingly defines 1 guacamole = 1 / (''Avocado's'' number) of moles. [https://dx.doi.org/10.1021/ar950245u Scientific paper with reference])
* [http://www.vendeka.org/ Vendeka.org] Home page for the use of the non-SI prefix ''vendeka'' to represent 10 to the power of 33, as in vendekabyte.

[[Category:Unit prefixes|*]]