Unit prefix

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 sizes are commonly formed by the use of such prefixes. The prefixes of the metric system, such as kilo and milli, represent multiplication by positive or negative powers of ten. In information technology it is common to use binary prefixes, which are based on 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

Metric prefixes in everyday use

• v
• t
• e

Prefix	Symbol	Factor	Power
tera	T	1000000000000	10^12
giga	G	1000000000	10^9
mega	M	1000000	10^6
kilo	k	1000	10^3
hecto	h	100	10^2
deca	da	10	10^1
(none)	(none)	1	10^0
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 the metric system precede a basic unit of measure to indicate a decadic multiple and 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). 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).

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 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 prefixes myria-, (from the Greek μύριοι, mýrioi), double- and demi-, denoting factors of 10000, 2 and 1⁄2 respectively, 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 CGPM conference in 1960. The prefix "myrio-" was an alternative spelling variant for "myria-", as proposed by Thomas Young.

Binary prefixes

Binary prefixes

Prefix	Symbol	Power
yobi	Yi	2^80
zebi	Zi	2^70
exbi	Ei	2^60
pebi	Pi	2^50
tebi	Ti	2^40
gibi	Gi	2^30
mebi	Mi	2^20
kibi	Ki	2^10

A binary prefix indicates multiplication by a power of two. The tenth power of 2 (2¹⁰) has the value 1024, which is close to 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 RAM capacity, kilobyte, megabyte and gigabyte customarily mean 1024 (2¹⁰), 1048576 (2²⁰) and 1073741824 (2³⁰) bytes respectively.

In the specifications of hard disk drive capacities and network transmission bit 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. (see Orin Safier v. Western Digital Corporation and Cho v. Seagate Technology (US) Holdings, Inc.). To protect themselves, some sellers write out the full term as "1000000".

With the aim of avoiding ambiguity the International Electrotechnical Commission (IEC) adopted new binary prefixes in 1998 (IEC 80000-13:2008 formerly subclauses 3.8 and 3.9 of 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 2¹⁰, 2²⁰ and 2³⁰ bytes respectively.

Unofficial prefixes

Although some of the following unofficial prefixes appear repeated on the internet, none are in use.

A metric prefix myria (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. The prefix bronto, as used in the term "brontobyte", has been used to represent anything from 10¹⁵ to 10²⁷ bytes, most often 10²⁷. In 2010, an online petition sought to establish hella- as the SI prefix for 10²⁷, a movement that began on the campus of UC Davis. The prefix, which has since appeared in the San Francisco Chronicle, Daily Telegraph, Wired and some other scientific magazines, was recognised by Google, in a non-serious fashion, in May 2010. Ian Mills, president of the Consultative Committee on Units, considered the chances of official adoption to be remote. The prefix geop and term "geopbyte" have been used in the information technology industry to refer to 10³⁰ bytes, following "brontobyte".

The ascending prefixes peta (1000⁵) and exa (1000⁶) are based on the Greek-derived numeric prefixes "penta" (5) and "hexa" (6). The largest prefixes zetta (1000⁷), and yotta (1000⁸) and, similarly, the descending prefixes zepto (1000⁻⁷) and yocto (1000⁻⁸) are derived from Latin "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 10²), 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.

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.

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) and revo (10⁻³³). 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 10⁻²⁷, groucho for 10⁻³⁰ (and therefore harpi for 10²⁷, grouchi for 10³⁰, zeppi for 10³³, gummi for 10³⁶, and chici for 10³⁹), all of these being references to the comedy act, the Marx Brothers.

Use for quantities of bits and bytes

Prefixes for decimal and binary multiples
Decimal Value SI 1000 10^3 k kilo 1000^2 10^6 M mega 1000^3 10^9 G giga 1000^4 10^12 T tera 1000^5 10^15 P peta 1000^6 10^18 E exa 1000^7 10^21 Z zetta 1000^8 10^24 Y yotta 1000^9 10^27 R ronna 1000^10 10^30 Q quetta	Binary Value IEC JEDEC 1024 2^10 Ki kibi K kilo 1024^2 2^20 Mi mebi M mega 1024^3 2^30 Gi gibi G giga 1024^4 2^40 Ti tebi T tera 1024^5 2^50 Pi pebi — 1024^6 2^60 Ei exbi — 1024^7 2^70 Zi zebi — 1024^8 2^80 Yi yobi — — —
v t e

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

1. "The names zepto and zetta suggest the digit seven [sept] (seventh power of 10³) 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 10³); 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." Resolution 4 of the 19th CGPM (1991)

References

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4. Brewster, David (1832). The Edinburgh Encyclopaedia. Vol. 12 (1st American ed.). Joseph and Edward Parker. Retrieved 2015-10-09.
5. "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" [The Law of 18 Germinal, Year 3: Decision to draw the fundamental unit metre on a platinum ruler. Nomenclature of "Republican measures". Resumption of the triangulation.]. L'Histoire Du Mètre [The history of the metre] (in French). histoire.du.metre.free.fr. Archived from the original on 2022-11-26. Retrieved 2015-10-12. 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.]
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8. Reimer, Jeremy (2006-06-30). "Western Digital settles drive size lawsuit". Ars Technica LLC. Retrieved 2012-02-18.
9. Seagate lawsuit concludes, settlement announced, bit-tech.net
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15. "BBC article suggesting that a "brontobyte" is 10²⁷ bytes".
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17. "Redirect".
18. "(undefined)". MacUser. 7: 362. 1991-02-16. Archived from the original on 2022-12-24. 1 brontobyte (1,000,000,000 megabytes)
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20. Moore, Matthew (2010-03-02). "Hella number: scientists call for new word for 1,000,000,000,000,000,000,000,000,000". The Telegraph. Archived from the original on 2022-12-24. Retrieved 2019-01-04. 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.
21. "Jargon Watch". Wired. Vol. 18, no. 6. June 2010. […] a proposed metric prefix […] useful for describing mega-measurements like Earth's mass (6 Hellagrams). A Facebook petition garnered 30000 signatures
22. "The Official Petition to Establish "Hella-" as the SI Prefix for 10^27". Facebook. Retrieved 2010-06-04.
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External links

• Moerner Lab Single-Molecule Research Page (Jokingly defines 1 guacamole = 1 / (Avocado's number) of moles. Scientific paper with reference)
• Vendeka.org Home page for the use of the non-SI prefix vendeka to represent 10 to the power of 33, as in vendekabyte.