A unit prefix is a specifier or mnemonic that is prepended to units 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.
The prefixes of the metric system precede a basic unit of measure to indicate a decadicmultiple and fraction of a unit. Each prefix has a unique symbol that is prepended to 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).[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 units hectare, decibel are also common. The unit prefixes are always considered to be part of the unit, so that, e.g., in exponentiation, 1km2 means one square kilometre, not one thousand square metres, and 1cm3 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-,[2][3][4] (from the Greek μύριοι, mýrioi), double- and demi-, denoting factors of 10000, 2 and 1⁄2 respectively,[5][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 11th CGPM conference in 1960. The prefix "myrio-" was an alternative spelling variant for "myria-", as proposed by Thomas Young. [3][4][6][7]
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.).[8][9] 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.[10]
Unofficial prefixes
Although some of the following unofficial prefixes appear repeated on the internet, none are in use.[11]
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.[12][13] The prefix bronto, 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 establish hella- as the SI prefix for 1027, a movement that began on the campus of UC Davis.[19][20] 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.[21][22][23] Ian Mills, president of the Consultative Committee on Units, considered the chances of official adoption to be remote.[24] The prefix geop and term "geopbyte" have been used in the information technology industry to refer to 1030 bytes, following "brontobyte".[14]
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[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 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.[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 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.[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) 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.[27]
↑ "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)
Related Research Articles
The byte is a unit of digital information that most commonly consists of eight bits. 1 byte (B) = 8 bits (bit). Historically, the byte was the number of bits used to encode a single character of text in a computer and for this reason it is the smallest addressable unit of memory in many computer architectures. To disambiguate arbitrarily sized bytes from the common 8-bit definition, network protocol documents such as the Internet Protocol refer to an 8-bit byte as an octet. Those bits in an octet are usually counted with numbering from 0 to 7 or 7 to 0 depending on the bit endianness.
A binary prefix is a unit prefix that indicates a multiple of a unit of measurement by an integer power of two. The most commonly used binary prefixes are kibi (symbol Ki, meaning 210 = 1024), mebi (Mi, 220 = 1048576), and gibi (Gi, 230 = 1073741824). They are most often used in information technology as multipliers of bit and byte, when expressing the capacity of storage devices or the size of computer files.
The gigabyte is a multiple of the unit byte for digital information. The prefix giga means 109 in the International System of Units (SI). Therefore, one gigabyte is one billion bytes. The unit symbol for the gigabyte is GB.
Giga- ( or ) is a unit prefix in the metric system denoting a factor of a short-scale billion or long-scale milliard (109 or 1,000,000,000). It has the symbol G.
The kilobyte is a multiple of the unit byte for digital information.
Kilo is a decimal unit prefix in the metric system denoting multiplication by one thousand (103). It is used in the International System of Units, where it has the symbol k, in lowercase.
The litre or liter is a metric unit of volume. It is equal to 1 cubic decimetre (dm3), 1000 cubic centimetres (cm3) or 0.001 cubic metres (m3). A cubic decimetre occupies a volume of 10 cm × 10 cm × 10 cm and is thus equal to one-thousandth of a cubic metre.
The megabyte is a multiple of the unit byte for digital information. Its recommended unit symbol is MB. The unit prefix mega is a multiplier of 1000000 (106) in the International System of Units (SI). Therefore, one megabyte is one million bytes of information. This definition has been incorporated into the International System of Quantities.
The International System of Units, internationally known by the abbreviation SI, is the modern form of the metric system and the world's most widely used system of measurement. It is the only system of measurement with official status in nearly every country in the world, employed in science, technology, industry, and everyday commerce. The SI system is coordinated by the International Bureau of Weights and Measures which is abbreviated BIPM from French: Bureau international des poids et mesures.
A metric prefix is a unit prefix that precedes a basic unit of measure to indicate a multiple or submultiple of the unit. All metric prefixes used today are decadic. Each prefix has a unique symbol that is prepended to any unit symbol. The prefix kilo-, for example, may be added to gram to indicate multiplication by one thousand: one kilogram is equal to one thousand grams. The prefix milli-, likewise, may be added to metre to indicate division by one thousand; one millimetre is equal to one thousandth of a metre.
The metric system is a decimal-based system of measurement. The current international standard for the metric system is the International System of Units, in which all units can be expressed in terms of seven base units: the metre (m), kilogram (kg), second (s), ampere (A), kelvin (K), mole (mol), and candela (cd). These can be made into larger or smaller units with the use of metric prefixes.
An order of magnitude is usually a factor of ten. Thus, four orders of magnitude is a factor of 10,000 or 104.
Myria- (symbol my) is a now obsolete decimal metric prefix denoting a factor of 104 (ten thousand). It originates from the Greek μύριοι (mýrioi) (myriad). The prefix was part of the original metric system adopted by France in 1795, but was not adopted when the SI prefixes were internationally adopted by the 11th CGPM conference in 1960.
IEEE 1541-2002 is a standard issued in 2002 by the Institute of Electrical and Electronics Engineers (IEEE) concerning the use of prefixes for binary multiples of units of measurement related to digital electronics and computing. IEEE 1541-2021 revises and supersedes IEEE 1541–2002, which is 'inactive'.
The JEDEC memory standards are the specifications for semiconductor memory circuits and similar storage devices promulgated by the Joint Electron Device Engineering Council (JEDEC) Solid State Technology Association, a semiconductor trade and engineering standardization organization.
In telecommunications, data transfer rate is the average number of bits (bitrate), characters or symbols (baudrate), or data blocks per unit time passing through a communication link in a data-transmission system. Common data rate units are multiples of bits per second (bit/s) and bytes per second (B/s). For example, the data rates of modern residential high-speed Internet connections are commonly expressed in megabits per second (Mbit/s).
This timeline of binary prefixes lists events in the history of the evolution, development, and use of units of measure that are germane to the definition of the binary prefixes by the International Electrotechnical Commission (IEC) in 1998, used primarily with units of information such as the bit and the byte.
In digital computing and telecommunications, a unit of information is the capacity of some standard data storage system or communication channel, used to measure the capacities of other systems and channels. In information theory, units of information are also used to measure information contained in messages and the entropy of random variables.
The history of the metric system began during the Age of Enlightenment with measures of length and weight derived from nature, along with their decimal multiples and fractions. The system became the standard of France and Europe within half a century. Other measures with unity ratios were added, and the system went on to be adopted across the world.
The following outline is provided as an overview of and topical guide to the metric system:
1 2 Brewster, David (1830). The Edinburgh Encyclopædia. Vol.12. Edinburgh, UK: William Blackwood, John Waugh, John Murray, Baldwin & Cradock, J. M. Richardson. p.494. Retrieved 2015-10-09.
1 2 Brewster, David (1832). The Edinburgh Encyclopaedia. Vol.12 (1st Americaned.). Joseph and Edward Parker. Retrieved 2015-10-09.
1 2 "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.]
↑ Dingler, Johann Gottfried (1823). Polytechnisches Journal (in German). Vol.11. Stuttgart, Germany: J.W. Gotta'schen Buchhandlung. Retrieved 2015-10-09.
↑ Shrivatav, P. N., ed. (1971). "Appendix B - XII Conversion Table". Gazetteer of India: Madhya Pradesh District Gazetteers - Indore. District Gazetteers Department, Madhya Pradesh, Bhopal. p.785.
↑ "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
Vendeka.org Home page for the use of the non-SI prefix vendeka to represent 10 to the power of 33, as in vendekabyte.
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