A unit prefix is a specifier or mnemonic that is prepended to units of measurement. Units of various sizes are commonly formed by the use of such prefixes. The prefixes of the metric system, such as kilo- and milli-, are well known examples. These represent multiplication by powers of ten. In the information technology field it is common to use binary prefixes, which are based on powers of two. A number of prefixes, have been used or proposed by various sources, but not adopted by any standards organisation. There are also prefixes which official support have been dropped.

Metric prefixes

The prefixes of the metric system precedes a basic unit of measure to indicate a decadic multiple or fraction of the unit. Each prefix has a unique symbol that is prepended to the unit symbol. Many of the prefixes date back to the introduction of the metric system in the 1790s others are more modern. There are currently twenty metric prefixes standardised by the International Bureau of Weights and Measures in resolutions dating from 1960 to 1991.^[1]

Distance marker on the Rhine: 36 (XXXVI) myriametres from Basel

Although formerly used, the SI disallows combining prefixes; the microkilogram or centimillimetre, for example, are not permitted. Prefixes corresponding to powers of one thousand are often recommended, however, such units as the hectopascal, hectare, decibel, centimetre and centilitre are commonly used. When units occur in exponentiation prefixes are considered part of the unit; 1 km², for example, 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 and may even be used with non-metric units, however, some combinations are more common than others. The choice of prefixes with a given unit is usually dictated by convenience of use, 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, standard combination are established. Prefixes for multiples greater than one thousand, for example, are rarely applied to the gram or metre.

Some of the prefixes used in older versions of the metric system but are no longer used. The prefixes myria,^[2][3] demi- and double-, denoting a factors of 10000 (from the Greek μύριοι, mýrioi), ¹⁄₂ and 2 respectively,^[4] 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 "myria-" has also been spelt "myrio-".^[5]

Binary prefixes

A binary prefix indicates multiplication by a power of two. The binary multiple 2¹⁰ (1024) is close to 1000. This has allowed the definition of a series of binary prefixes of comparable size to the metric kilo-, mega-, giga- et seq. Prefixes based on this series are commonly used in the information technology field prepended to the units of digital information, the bit and the byte.

Units of information are outside of the International System of Units and so the International Bureau of Weights and Measures makes no comment as to the prefixes appended to them. Computer professionals have historically used the same spelling, pronunciation and symbols for the binary series in the description of computer memory (with the exception the symbol for the binary 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, on the other hand, decimal prefixes, consistent with the metric system, are used. For example, a 500-gigabyte hard drive holds 500000000000 bytes, and a 100-megabit-per-second Ethernet connexion transfers data at 100000000 bits per second. The amiguity has led to some confusion and even of 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.).^[6][7] 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. would only be used in the decimal sense, even when referring to data storage capacities: kilobyte and megabyte would denote one thousand and one million bytes respectively (consistent with the metric system), while new terms such as kibibyte, mebibyte and gibibyte, abbreviated KiB, MiB and GiB, would denote 2¹⁰, 2²⁰ and 2³⁰ bytes respectively.^[8] However, as of 2012 adoption of the new terms has been slow and usage has been limited.^[9]

Unofficial prefixes

There are many unofficial or fabricated metric prefixes circulating the internet, especially for values smaller than 10⁻²⁴ or larger than 10²⁴. ^{[10] [11]} One unofficial prefix is bronto-, which has been used in the term brontobyte to represent anything from 10¹⁵ to 10²⁷ bytes.^{[12][13][14][15][16]} SI has already produced standard prefixes for 10¹⁵ (peta), 10²¹ (zetta) and 10²⁴ (yotta).

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.^[17][18] The prefix, which has since appeared in the San Francisco Chronicle, Daily Telegraph, Wired and some other scientific magazines, was recognized by Google, in a non-serious fashion, in May 2010.^[19][20][21] Ian Mills, president of the Consultative Committee on Units, considers the chances of official adoption to be remote.^[22]

A more serious proposal from 2003 by Jim Blower follows the already existing naming convention based on reverse lettering in the alphabet:^[23][24]

• zetta-, yotta-, xona-, weka-, vunda-, uda-, treda-, sorta-, rinta-, quexa-, pepta-, ocha-, nena-, minga-, luma-, ...
• zepto-, yocto-, xonto-, wekto-, vunkto-, unto-, trekto-, sotro-, rimto-, quekto-, pekro-, otro-, nekto-, mikto-, lunto-, ...

The z and y on the extreme prefixes zepto-, yocto- and zetta-, yotta- suggest the start of a series backwards through the alphabet. Following the initial letters are distorted Greek numerals (h)epta- and octa-. This pattern has been extended further, with the terms xenno-, weko-, vendeko- and xenna-, weka-, vendeka- from Greek ennea-, deka-, endeka-; sometimes a t is added for greater differentiation, though both xento-, wekto- etc. and xenta-, wekta- etc. have been proposed.^[25]

References

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.