Aunit prefix is a specifier or mnemonic that is added to the beginning of aunit of measurement to indicate multiples or fractions of the units. Units of varioussizes are commonly formed by the use of suchprefixes. Theprefixes of the metric system, such askilo andmilli, represent multiplication by positive or negativepowers of ten. In information technology it is common to usebinary prefixes, which are based onpowers of two. Historically, many prefixes have been used or proposed by various sources, but only a narrow set has been recognised by standards organisations.
| Prefix | Symbol | Factor | Power |
|---|---|---|---|
| tera | T | 1000000000000 | 1012 |
| giga | G | 1000000000 | 109 |
| mega | M | 1000000 | 106 |
| kilo | k | 1000 | 103 |
| hecto | h | 100 | 102 |
| deca | da | 10 | 101 |
| (none) | (none) | 1 | 100 |
| deci | d | 0.1 | 10−1 |
| centi | c | 0.01 | 10−2 |
| milli | m | 0.001 | 10−3 |
| micro | μ | 0.000001 | 10−6 |
| nano | n | 0.000000001 | 10−9 |
| pico | p | 0.000000000001 | 10−12 |
The prefixes of themetric system precede a basic unit of measure to indicate adecadicmultiple andfraction 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. TheInternational Bureau of Weights and Measures has standardised twenty metric prefixes in resolutions dating from 1960 to 1991 for use with theInternational System of Units (SI).[1] In addition to those listed in the everyday-use table, the SI includes standardised prefixes for 1015 (peta), 1018 (exa), 1021 (zetta), 1024 (yotta), 1027 (ronna), and 1030 (quetta); and for 10−15 (femto), 10−18 (atto), 10−21 (zepto), 10−24 (yocto), 10−27 (ronto), and 10−30 (quecto).
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 unitshectare,decibel are also common. The unit prefixes are always considered to be part of the unit, so that, e.g., inexponentiation, 1 km2 means one square kilometre, not one thousand square metres, and 1 cm3 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 prefixesmyria-,[2][3][4] (from the Greekμύριοι,mýrioi),double- anddemi-, denoting factors of10000, 2 and1⁄2 respectively,[5] 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 11thCGPM conference in 1960. The prefix "myrio-" was an alternative spelling variant for "myria-", as proposed byThomas Young.[3][4][6][7]
| Decimal | Binary | ||||||
|---|---|---|---|---|---|---|---|
| Value | SI | Value | IEC | JEDEC | |||
| 1000 | k | kilo | 1024 | Ki | kibi | K | kilo |
| 10002 | M | mega | 10242 | Mi | mebi | M | mega |
| 10003 | G | giga | 10243 | Gi | gibi | G | giga |
| 10004 | T | tera | 10244 | Ti | tebi | T | tera |
| 10005 | P | peta | 10245 | Pi | pebi | — | |
| 10006 | E | exa | 10246 | Ei | exbi | — | |
| 10007 | Z | zetta | 10247 | Zi | zebi | — | |
| 10008 | Y | yotta | 10248 | Yi | yobi | — | |
| 10009 | R | ronna | 10249 | Ri | robi | — | |
| 100010 | Q | quetta | 102410 | Qi | quebi | — | |
Abinary prefix indicates multiplication by a power of two. The tenth power of 2 (210) has the value1024, which is close to1000. This has prompted the use of the metric prefixeskilo,mega, andgiga to also denote the powers of 1024 which is common in information technology with the unit of digital information, thebyte.
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 ofcomputer memory, although the symbol forkilo is often capitalised. For example, in citations of main memory orRAM capacity,kilobyte,megabyte andgigabyte customarily mean1024 (210),1048576 (220) and1073741824 (230) bytes respectively.
In the specifications ofhard disk drive capacities and network transmissionbit rates, 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 220 or 230 and considered themselves shortchanged by the seller. (seeOrin Safier v. Western Digital Corporation andCho v. Seagate Technology (US) Holdings, Inc.).[8][9] To protect themselves, some sellers write out the full term as "1000000".
With the aim of avoiding ambiguity theInternational Electrotechnical Commission (IEC) adopted new binary prefixes in 1998 (IEC 80000-13:2008 formerly subclauses 3.8 and 3.9 ofIEC 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 andmegabyte denote one thousand and one million bytes respectively (consistent with the metric system), while terms such askibibyte,mebibyte andgibibyte, with symbols KiB, MiB and GiB, denote 210, 220 and 230 bytes respectively.[10]
Although some of the following unofficial prefixes appear repeated on the internet, none are in use.[11]
A metric prefixmyria (abbreviation "my"), for 10,000, was deprecated in 1960. Before the adoption of ronna and quetta for 1027 and 1030 and ronto and quecto for 10−27 and 10−30 in November 2022, many personal, and sometimes facetious, proposals for additional metric prefixes were formulated.[12][13] The prefixbronto, as used in the term "brontobyte", has been used to represent anything from 1015 to 1027 bytes, most often 1027.[14][15][16][17][18] In 2010, an online petition sought to establishhella- as the SI prefix for 1027, a movement that began on the campus ofUC Davis.[19][20] The prefix, which has since appeared in theSan Francisco Chronicle,Daily Telegraph,Wired and some other scientific magazines, was recognised byGoogle, in anon-serious fashion, in May 2010.[21][22][23] Ian Mills, president of the Consultative Committee on Units, considered the chances of official adoption to be remote.[24] The prefixgeop and term "geopbyte" have been used in theinformation technology industry to refer to 1030 bytes, following "brontobyte".[14]
The ascending prefixespeta (10005) andexa (10006) are based on the Greek-derivednumeric prefixes "penta" (5) and "hexa" (6). The largest prefixeszetta (10007), andyotta (10008) and, similarly, the descending prefixeszepto (1000−7) andyocto (1000−8) are derived from Latin[25] "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 ascendinghepto (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 102), the same applied to "s" from previously proposed descendingsepto (i.e. SI unit "s", seconds), while "o" forocto was problematic since a symbol "o" could be confused with zero.[nb 1]
Before the adoption of new prefixes in 2022, several personal proposals had been made for extending the series of prefixes, with ascending terms such asxenna,weka,vendeka (from Greek "ennea" (9), "deka" (10), "endeka" (11)) and descending terms such asxono,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 asdeca,hecto,kilo vs.deci,centi,milli.[26]
In 2001, a few unofficial prefixes appeared on the Internet:hepa (1021),ento (10−21),otta (1024),fito (10−24),nea (1027),syto (10−27),dea (1030),tredo (10−30),una (1033) andrevo (10−33). The Oxford professorJeffrey K. Aronson has suggested extending beyondzetta/zepto andyotta/yocto withxenta/xenno,wekta/weko,vendeka/vendeko, andudeka/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 withXona, Weka, Vunda, Uda, Treda, Sorta, ... Another proposal forxenta/xona isnovetta, from the Italian "nove" (or "nine"). In 1993, Morgan Burke proposed, as a joke,harpo for 10−27,groucho for 10−30 (and thereforeharpi for 1027,grouchi for 1030,zeppi for 1033,gummi for 1036, andchici for 1039), all of these being references to the comedy act, theMarx Brothers.[27]
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.[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.]
1 brontobyte (1,000,000,000 megabytes)
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.
[…] a proposed metric prefix […] useful for describing mega-measurements like Earth's mass (6 Hellagrams). A Facebook petition garnered30000 signatures