A metric prefix is a

^{2} is km × km, not km × m.

^{2} means one ^{3} means two cubic

^{7}) and ^{−14}).

^{−4}), which was contracted to "dimi-" and standardised in France up to 1961.
There are no more letters of the Latin alphabet available for new prefixes (all the unused letters are already used for units). As such, Richard J. C. Brown (who proposed the prefixes adopted for 10^{±27} and 10^{±30}) has proposed a reintroduction of compound prefixes (e.g. ''kiloquetta-'' for 10^{33}) if a driver for prefixes at such scales ever materialises, with a restriction that the last prefix must always be ''quetta-'' or ''quecto-''. This usage is not currently approved by the BIPM.

^{9}) and trillion (10^{12}), respectively, for large quantities, typically currency and population.
The

^{10} = ,
* mebi (Mi) = 2^{20} = ^{2} = ,
* gibi (Gi) = 2^{30} = ^{3} = ,
etc.

International Bureau of Weights and Measures

(BIPM)

SI prefixes at BIPM

* ttp://physics.nist.gov/cuu/Units/binary.html US NIST ''Definitions of the SI units: The binary prefixes'' {{Portal bar, Physics Numeral systems

unit prefix 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 ...

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
The kilogram (also kilogramme) is the unit of mass in the International System of Units (SI), having the unit symbol kg. It is a widely used measure in science, engineering and commerce worldwide, and is often simply called a kilo colloquiall ...

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.
Decimal multiplicative prefixes have been a feature of all forms of the metric system
The metric system is a system of measurement that succeeded the decimalised system based on the metre that had been introduced in France in the 1790s. The historical development of these systems culminated in the definition of the Intern ...

, with six of these dating back to the system's introduction in the 1790s. Metric prefixes have also been used with some non-metric units. The SI prefixes are metric prefixes that were standardised for use in the International System of Units
The International System of Units, known by the international abbreviation SI in all languages and sometimes Pleonasm#Acronyms and initialisms, pleonastically as the SI system, is the modern form of the metric system and the world's most wid ...

(SI) by the International Bureau of Weights and Measures
The International Bureau of Weights and Measures (french: Bureau international des poids et mesures, BIPM) is an intergovernmental organisation, through which its 59 member-states act together on measurement standards in four areas: chemistr ...

(BIPM) in resolutions dating from 1960 to 2022. Since 2009, they have formed part of the ISO/IEC 80000 standard. They are also used in the Unified Code for Units of Measure The Unified Code for Units of Measure (UCUM) is a system of codes for unambiguously representing measurement units. Its primary purpose is machine-to-machine communication rather than communication between humans.
The code set includes all units de ...

(UCUM).
List of SI prefixes

The BIPM specifies twenty-four prefixes for the International System of Units (SI). First uses of prefixes in SI date back to definition of kilogram after the French Revolution at the end of the 18th century. Several more prefixes came into use by the 1947IUPAC
The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...

14th International Conference of Chemistry before being officially adopted for the first time in 1960.
The most recent prefixes adopted were ''ronna-'', ''quetta-'', ''ronto-'', and ''quecto-'' in 2022, after a proposal from British metrologist Richard J. C. Brown. The large prefixes ''ronna-'' and ''quetta-'' were adopted in anticipation of needs from data science, and because unofficial prefixes that did not meet SI requirements were already circulating. The small prefixes were added as well even without such a driver in order to maintain symmetry. After these adoptions, all Latin letters have now been used for prefixes or units.
Rules

* Each prefix name has a symbol that is used in combination with the symbols for units of measure. For example, the symbol for ''kilo-'' is k, and is used to produce km, kg, and kW, which are the SI symbols for kilometre, kilogram, and kilowatt, respectively. Except for the early prefixes of ''kilo-'', ''hecto-'', and ''deca-'', the symbols for the prefixes for multiples are uppercase letters, and those for the prefixes for submultiples are lowercase letters. * All of the metric prefix symbols are made from upper- and lower-case Latin letters except for the symbol for ''micro'', which is uniquely aGreek letter
The Greek alphabet has been used to write the Greek language since the late 9th or early 8th century BCE. It is derived from the earlier Phoenician alphabet, and was the earliest known alphabetic script to have distinct letters for vowels as ...

"".
* Like the numbers they combine with, SI units and unit symbols are never shown in ''italics''. The prefixes and their symbols are always prefixed to the symbol without any intervening space or punctuation. This distinguishes a prefixed unit symbol from the product of unit symbols, for which a space or mid-height dot as separator is required. So, for instance, while 'ms' means millisecond, 'm s' or 'm·s' means metre second.
* Prefixes corresponding to an integer power of one thousand are generally preferred, and the prefixes for tens (deci-, deca-) and hundreds (cent-, hecto-) are disfavoured. Hence 100 m is preferred over 1 hm (hectometre) or 10 dam (decametres). The prefixes ''deci-'' and ''centi-'', and less frequently ''hecto-'' and ''deca-'', are commonly used for everyday purposes; the centimetre (cm) is especially common. Some modern building codes require that the millimetre be used in preference to the centimetre, because "use of centimetres leads to extensive usage of decimal points and confusion". Deprecated prefixes are also used to create metric units corresponding to older conventional units, for example hectares and hectopascal
The pascal (symbol: Pa) is the unit of pressure in the International System of Units (SI), and is also used to quantify internal pressure, stress, Young's modulus, and ultimate tensile strength. The unit, named after Blaise Pascal, is defin ...

s.
* Prefixes may not be used in combination on a single symbol. This includes the case of the base unit kilogram, which already contains a prefix. For example, milligram (mg) is used instead of microkilogram (μkg).
* In the arithmetic of measurements having units, the units are treated as multiplicative factors to values. In the product of multiple units, each individual unit prefix must be evaluated as a separate numeric multiplier and then combined with the others.
* A prefix symbol attached to a unit symbol is included when the unit is raised to a power. For example, kmUsage

Examples

* The mass of anelectron
The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary particles because they have no ...

is about 1 rg (rontogram).
* The mass of 1 litre of water is about 1 kg (kilogram).
* The mass of the Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...

is about 6 Rg (ronnagrams).
* The mass of Jupiter
Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but slightly less than one-thousandth t ...

is about 2 Qg (quettagrams).
Examples of powers of units with metric prefixes

* 1 kmsquare kilometre
Square kilometre ( International spelling as used by the International Bureau of Weights and Measures) or square kilometer (American spelling), symbol km2, is a multiple of the square metre, the SI unit of area or surface area.
1 km2 is e ...

, or the area
Area is the quantity that expresses the extent of a region on the plane or on a curved surface. The area of a plane region or ''plane area'' refers to the area of a shape or planar lamina, while '' surface area'' refers to the area of an open ...

of a square
In Euclidean geometry, a square is a regular quadrilateral, which means that it has four equal sides and four equal angles (90- degree angles, π/2 radian angles, or right angles). It can also be defined as a rectangle with two equal-lengt ...

of by . In other words, an area of square metre
The square metre ( international spelling as used by the International Bureau of Weights and Measures) or square meter ( American spelling) is the unit of area in the International System of Units (SI) with symbol m2. It is the area of a square ...

s and not square metre
The square metre ( international spelling as used by the International Bureau of Weights and Measures) or square meter ( American spelling) is the unit of area in the International System of Units (SI) with symbol m2. It is the area of a square ...

s.
* 2 Mmmegametre
The following are examples of orders of magnitude for different lengths.
__TOC__
Overview
Detailed list
To help compare different orders of magnitude, the following list describes various lengths between 1.6 \times 10^ metres and 10 ...

s, or the volume
Volume is a measure of occupied three-dimensional space. It is often quantified numerically using SI derived units (such as the cubic metre and litre) or by various imperial or US customary units (such as the gallon, quart, cubic inch). The ...

of two cube
In geometry, a cube is a three-dimensional solid object bounded by six square faces, facets or sides, with three meeting at each vertex. Viewed from a corner it is a hexagon and its net is usually depicted as a cross.
The cube is the onl ...

s of by by or , and not cubic metre
The cubic metre (in Commonwealth English and international spelling as used by the International Bureau of Weights and Measures) or cubic meter (in American English) is the unit of volume in the International System of Units (SI). Its symbol is ...

s ().
Examples with prefixes and powers

* × = × = = . * + = + = . * = = = 0.05 m. * = = = = = . * 3 MW = = 3 × = .Typesetting

There is an oldextended ASCII
Extended ASCII is a repertoire of character encodings that include (most of) the original 96 ASCII character set, plus up to 128 additional characters. There is no formal definition of "extended ASCII", and even use of the term is sometimes critic ...

symbol ("", Unicode
Unicode, formally The Unicode Standard,The formal version reference is is an information technology standard for the consistent encoding, representation, and handling of text expressed in most of the world's writing systems. The standard, whi ...

U+00B5) for ''micro'' for use when the Greek letter "" (U+03BC) is unavailable.
The LaTeX typesetting system features an ''SIunitx'' package in which the units of measurement are spelled out, for example, `\SI`

formats as "3 THz".
Application to units of measurement

The use of prefixes can be traced back to the introduction of the metric system in the 1790s, long before the 1960 introduction of the SI. The prefixes, including those introduced after 1960, are used with any metric unit, whether officially included in the SI or not (e.g., millidyne and milligauss). Metric prefixes may also be used with some non-metric units, but not, for example, with the non-SI units of time.Metric units

Mass

The unitskilogram
The kilogram (also kilogramme) is the unit of mass in the International System of Units (SI), having the unit symbol kg. It is a widely used measure in science, engineering and commerce worldwide, and is often simply called a kilo colloquiall ...

, gram
The gram (originally gramme; SI unit symbol g) is a unit of mass in the International System of Units (SI) equal to one one thousandth of a kilogram.
Originally defined as of 1795 as "the absolute weight of a volume of pure water equal ...

, milligram
The kilogram (also kilogramme) is the unit of mass in the International System of Units (SI), having the unit symbol kg. It is a widely used measure in science, engineering and commerce worldwide, and is often simply called a kilo colloquiall ...

, microgram, and smaller are commonly used for measurement of mass
Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different eleme ...

. However, megagram, gigagram, and larger are rarely used; tonne
The tonne ( or ; symbol: t) is a unit of mass equal to 1000 kilograms. It is a non-SI unit accepted for use with SI. It is also referred to as a metric ton to distinguish it from the non-metric units of the short ton (United States ...

s (and kilotonnes, megatonnes, etc.) or scientific notation
Scientific notation is a way of expressing numbers that are too large or too small (usually would result in a long string of digits) to be conveniently written in decimal form. It may be referred to as scientific form or standard index form, ...

are used instead. The megagram does not share the risk of confusion that the tonne has with other units with the name "ton".
The kilogram is the only coherent unit of the International System of Units
The International System of Units, known by the international abbreviation SI in all languages and sometimes Pleonasm#Acronyms and initialisms, pleonastically as the SI system, is the modern form of the metric system and the world's most wid ...

that includes a metric prefix.
Volume

Thelitre
The litre (international spelling) or liter (American English spelling) (SI symbols L and l, other symbol used: ℓ) is a metric unit of volume. It is equal to 1 cubic decimetre (dm3), 1000 cubic centimetres (cm3) or 0.001 cubic metre ( ...

(equal to a cubic decimetre), millilitre (equal to a cubic centimetre), microlitre, and smaller are common. In Europe, the centilitre is often used for liquids, and the decilitre is used less frequently. Bulk agricultural products, such as grain, beer and wine, often use the hectolitre (100 litres).
Larger volumes are usually denoted in kilolitres, megalitres or gigalitres, or else in cubic metres (1 cubic metre = 1 kilolitre) or cubic kilometres (1 cubic kilometre = 1 teralitre). For scientific purposes, the cubic metre is usually used.
Length

The kilometre, metre, centimetre, millimetre, and smaller units are common. The decimetre is rarely used. The micrometre is often referred to by the older non-SI name ''micron
The micrometre ( international spelling as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer ( American spelling), also commonly known as a micron, is a unit of length in the International System of U ...

''. In some fields, such as chemistry
Chemistry is the scientific study of the properties and behavior of matter. It is a natural science that covers the elements that make up matter to the compounds made of atoms, molecules and ions: their composition, structure, propertie ...

, the ångström
The angstromEntry "angstrom" in the Oxford online dictionary. Retrieved on 2019-03-02 from https://en.oxforddictionaries.com/definition/angstrom.Entry "angstrom" in the Merriam-Webster online dictionary. Retrieved on 2019-03-02 from https://www.m ...

(0.1 nm) has been used commonly instead of the nanometre. The femtometre, used mainly in particle physics, is sometimes called a fermi. For large scales, megametre, gigametre, and larger are rarely used. Instead, ad hoc non-metric units are used, such as the solar radius, astronomical unit
The astronomical unit (symbol: au, or or AU) is a unit of length, roughly the distance from Earth to the Sun and approximately equal to or 8.3 light-minutes. The actual distance from Earth to the Sun varies by about 3% as Earth orbi ...

s, light year
A light-year, alternatively spelled light year, is a large unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (), or 5.88 trillion miles ().One trillion here is taken to be 1012 ...

s, and parsec
The parsec (symbol: pc) is a unit of length used to measure the large distances to astronomical objects outside the Solar System, approximately equal to or (au), i.e. . The parsec unit is obtained by the use of parallax and trigonometry, a ...

s; the astronomical unit is mentioned in the SI standards as an accepted non-SI unit.
Time

Prefixes for the SI standard unitsecond
The second (symbol: s) is the unit of time in the International System of Units (SI), historically defined as of a day – this factor derived from the division of the day first into 24 hours, then to 60 minutes and finally to 60 seconds e ...

are most commonly encountered for quantities less than one second. For larger quantities, the system of minute
The minute is a unit of time usually equal to (the first sexagesimal fraction) of an hour, or 60 seconds. In the UTC time standard, a minute on rare occasions has 61 seconds, a consequence of leap seconds (there is a provision to insert a neg ...

s (60 seconds), hour
An hour ( symbol: h; also abbreviated hr) is a unit of time conventionally reckoned as of a day and scientifically reckoned between 3,599 and 3,601 seconds, depending on the speed of Earth's rotation. There are 60 minutes in an hour, and 24 ...

s (60 minutes) and day
A day is the time period of a full rotation of the Earth with respect to the Sun. On average, this is 24 hours, 1440 minutes, or 86,400 seconds. In everyday life, the word "day" often refers to a solar day, which is the length between two so ...

s (24 hours) is accepted for use with the SI and more commonly used. When speaking of spans of time, the length of the day is usually standardised to seconds so as not to create issues with the irregular leap second
A leap second is a one-second adjustment that is occasionally applied to Coordinated Universal Time (UTC), to accommodate the difference between precise time (International Atomic Time (TAI), as measured by atomic clocks) and imprecise observ ...

.
Larger multiples of the second such as kiloseconds and megaseconds are occasionally encountered in scientific contexts, but are seldom used in common parlance. For long-scale scientific work, particularly in astronomy
Astronomy () is a natural science that studies celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and evolution. Objects of interest include planets, moons, stars, nebulae, gala ...

, the Julian year or ''annum'' is a standardised variant of the year
A year or annus is the orbital period of a planetary body, for example, the Earth, moving in its orbit around the Sun. Due to the Earth's axial tilt, the course of a year sees the passing of the seasons, marked by change in weather, the ...

, equal to exactly seconds ( days). The unit is so named because it was the average length of a year in the Julian calendar
The Julian calendar, proposed by Roman consul Julius Caesar in 46 BC, was a reform of the Roman calendar. It took effect on , by edict. It was designed with the aid of Greek mathematicians and astronomers such as Sosigenes of Alexandr ...

. Long time periods are then expressed by using metric prefixes with the annum, such as megaannum or gigaannum.
Angle

The SI unit of angle is the radian, but degrees, as well as arc-minutes and arc-seconds, see some scientific use.Temperature

Common practice does not typically use the flexibility allowed by official policy in the case of the degree Celsius (°C). NIST states: "Prefix symbols may be used with the unit symbol °C and prefix names may be used with the unit name ''degree Celsius''. For example, 12 m°C (12 millidegrees Celsius) is acceptable." In practice, it is more common for prefixes to be used with the kelvin when it is desirable to denote extremely large or small absolute temperatures or temperature differences. Thus, temperatures of star interiors may be given in units of MK (megakelvins), and molecular cooling may be described in mK (millikelvins).Energy

In use thejoule
The joule ( , ; symbol: J) is the unit of energy in the International System of Units (SI). It is equal to the amount of work done when a force of 1 newton displaces a mass through a distance of 1 metre in the direction of the force applied ...

and kilojoule are common, with larger multiples seen in limited contexts. In addition, the kilowatt-hour, a composite unit formed from the kilowatt and hour, is often used for electrical energy; other multiples can be formed by modifying the prefix of watt (e.g. terawatt-hour).
There exist a number of definitions for the non-SI unit, the calorie
The calorie is a unit of energy. For historical reasons, two main definitions of "calorie" are in wide use. The large calorie, food calorie, or kilogram calorie was originally defined as the amount of heat needed to raise the temperature of o ...

. There are gram calories and kilogram calories. One kilogram calorie, which equals one thousand gram calories, often appears capitalised and without a prefix (i.e. ''Cal'') when referring to " dietary calories" in food. It is common to apply metric prefixes to the gram calorie, but not to the kilogram calorie: thus, 1 kcal = 1000 cal = 1 Cal.
Non-metric units

Metric prefixes are widely used outside the metric SI system. Common examples include the megabyte and thedecibel
The decibel (symbol: dB) is a relative unit of measurement equal to one tenth of a bel (B). It expresses the ratio of two values of a power or root-power quantity on a logarithmic scale. Two signals whose levels differ by one decibel have a ...

. Metric prefixes rarely appear with imperial or US units except in some special cases (e.g., microinch, kilofoot, kilopound
A kip is a US customary unit of force. It equals 1000 pounds-force, and is used primarily by structural engineers to indicate forces where the value represented in pound-force is inefficient. Although uncommon, it is occasionally also considered ...

). They are also used with other specialised units used in particular fields (e.g., megaelectronvolt
In physics, an electronvolt (symbol eV, also written electron-volt and electron volt) is the measure of an amount of kinetic energy gained by a single electron accelerating from rest through an electric potential difference of one volt in va ...

, gigaparsec
The parsec (symbol: pc) is a unit of length used to measure the large distances to astronomical objects outside the Solar System, approximately equal to or (au), i.e. . The parsec unit is obtained by the use of parallax and trigonometry, and ...

, millibarn, kilodalton). In astronomy, geology, and palaeontology, the year
A year or annus is the orbital period of a planetary body, for example, the Earth, moving in its orbit around the Sun. Due to the Earth's axial tilt, the course of a year sees the passing of the seasons, marked by change in weather, the ...

, with symbol a (from the Latin ''annus''), is commonly used with metric prefixes: ka, Ma, and Ga.
Official policies about the use of SI prefixes with non-SI units vary slightly between the International Bureau of Weights and Measures (BIPM) and the American National Institute of Standards and Technology
The National Institute of Standards and Technology (NIST) is an agency of the United States Department of Commerce whose mission is to promote American innovation and industrial competitiveness. NIST's activities are organized into physical sc ...

(NIST). For instance, the NIST advises that 'to avoid confusion, prefix symbols (and prefix names) are not used with the time-related unit symbols (names) min (minute), h (hour), d (day); nor with the angle-related symbols (names) ° (degree), ′ (minute), and ″ (second), whereas the BIPM adds information about the use of prefixes with the symbol ''as'' for arcsecond when they state: "However astronomers use milliarcsecond, which they denote mas, and microarcsecond, μas, which they use as units for measuring very small angles."
Non-standard prefixes

Obsolete metric prefixes

Some of the prefixes formerly used in the metric system have fallen into disuse and were not adopted into the SI. The decimal prefix for ten thousand, ''myria-
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, bu ...

'' (sometimes spelled '' myrio-''), and the early binary prefixes
A binary prefix is a unit prefix for multiples of units. It is most often used in data processing, data transmission, and digital information, principally in association with the bit and the byte, to indicate multiplication by a power ...

''double-'' (2×) and ''demi-'' (×) were parts of the original metric system adopted by France in 1795, but were not retained when the SI prefixes were internationally adopted by the 11th CGPM conference in 1960.
Other metric prefixes used historically include hebdo-
Hebdo- (symbol H) is an obsolete decimal metric prefix equal to 107. It is derived from the Greek ''hebdοmos'' ( el, ἕβδομος) meaning ''seventh''.
The definition of one ''hebdomometre'' or ''hebdometre'' as was originally proposed by R ...

(10micri- ''Micri-'' (unit symbol ''mc-'') is an archaic non- SI decimal metric prefix for 10−14.
It was proposed as a prefix for the CGS-unit of energy, the erg. The ''micrierg'' was proposed in 1922 by William Draper Harkins as a unit of energy equatin ...

(10Double prefixes

Double prefixes have been used in the past, such as ''micromillimetres'' or ''millimicrons'' (now nanometres), ''micromicrofarads'' (μμF; now picofarads, pF), ''kilomegatonnes'' (now gigatonnes), ''hectokilometres'' (now 100 kilometres) and the derived adjective ''hectokilometric'' (typically used for qualifying the fuel consumption measures). These are not compatible with the SI. Other obsolete double prefixes included "decimilli-" (10Similar symbols and abbreviations

In written English, the symbol ''K'' is often used informally to indicate a multiple of thousand in many contexts. For example, one may talk of a ''40K salary'' (), or call theYear 2000 problem
The year 2000 problem, also known as the Y2K problem, Y2K scare, millennium bug, Y2K bug, Y2K glitch, Y2K error, or simply Y2K refers to potential computer errors related to the formatting and storage of calendar data for dates in and after ...

the ''Y2K problem''. In these cases, an uppercase K is often used with an implied unit (although it could then be confused with the symbol for the kelvin temperature unit if the context is unclear). This informal postfix is read or spoken as "thousand" or "grand", or just "k".
The financial and general news media mostly use m or M, b or B, and t or T as abbreviations for million, billion (10medical
Medicine is the science and practice of caring for a patient, managing the diagnosis, prognosis, prevention, treatment, palliation of their injury or disease, and promoting their health. Medicine encompasses a variety of health care pr ...

and automotive fields in the United States use the abbreviations ''cc'' or ''ccm'' for cubic centimetres. One cubic centimetre is equal to one millilitre.
For nearly a century, engineers used the abbreviation ''MCM'' to designate a "thousand circular mils" in specifying the cross-sectional area of large electrical cables. Since the mid-1990s, ''kcmil
A circular mil is a unit of area, equal to the area of a circle with a diameter of one mil (one thousandth of an inch or ). It corresponds to approximately . It is a unit intended for referring to the area of a wire with a circular cross section. ...

'' has been adopted as the official designation of a thousand circular mils, but the designation ''MCM'' still remains in wide use. A similar system is used in natural gas sales in the United States: ''m'' (or ''M'') for thousands and ''mm'' (or ''MM'') for millions of British thermal unit
The British thermal unit (BTU or Btu) is a unit of heat; it is defined as the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit. It is also part of the United States customary units. The modern SI ...

s or therms, and in the oil industry, where ''MMbbl'' is the symbol for "millions of barrels". This usage of the capital letter ''M'' for "thousand" is from Roman numerals
Roman numerals are a numeral system that originated in ancient Rome and remained the usual way of writing numbers throughout Europe well into the Late Middle Ages. Numbers are written with combinations of letters from the Latin alphabet, ...

, in which ''M'' means 1000.
Binary prefixes

The original metric system adopted by France in 1795 included the twobinary prefix
A binary prefix is a unit prefix for multiples of units. It is most often used in data processing, data transmission, and digital information, principally in association with the bit and the byte, to indicate multiplication by a power of ...

es ''double-'' (2×) and ''demi-'' (×). However, they were not retained when the SI prefixes were internationally adopted by the 11th CGPM conference in 1960.
In some fields of information technology, it has been common to designate non-decimal multiples based on powers of 1024, rather than 1000, for some SI prefixes (''kilo-'', ''mega-'', ''giga-''), contrary to the definitions in the International System of Units
The International System of Units, known by the international abbreviation SI in all languages and sometimes Pleonasm#Acronyms and initialisms, pleonastically as the SI system, is the modern form of the metric system and the world's most wid ...

(SI). The SI does not permit the metric prefixes to be used in this conflicting sense. This practice was once sanctioned by some industry associations, including JEDEC. The International Electrotechnical Commission
The International Electrotechnical Commission (IEC; in French: ''Commission électrotechnique internationale'') is an international standards organization that prepares and publishes international standards for all electrical, electronic and ...

(IEC) standardised the system of binary prefix
A binary prefix is a unit prefix for multiples of units. It is most often used in data processing, data transmission, and digital information, principally in association with the bit and the byte, to indicate multiplication by a power of ...

es (''kibi-'', ''mebi-'', ''gibi-'', etc.) for this purpose.The names and symbols of the binary prefix
A binary prefix is a unit prefix for multiples of units. It is most often used in data processing, data transmission, and digital information, principally in association with the bit and the byte, to indicate multiplication by a power of ...

es standardised by the IEC include:
* kibi (Ki) = 2See also

*Binary prefix
A binary prefix is a unit prefix for multiples of units. It is most often used in data processing, data transmission, and digital information, principally in association with the bit and the byte, to indicate multiplication by a power of ...

* Engineering notation
* E1 series (preferred numbers)
* Indian numbering system
The Indian numbering system is used in all South Asian countries (Bangladesh, Bhutan, India, Maldives, Nepal, Pakistan, Sri Lanka and Afghanistan) to express large numbers. The terms ''lakh'' or 1,00,000 (one hundred thousand, written as ''10 ...

* International vocabulary of metrology
* ISO/IEC 80000
* Names of large numbers
* Names of small numbers
* Number names
* Numeral prefix
* Order of magnitude
An order of magnitude is an approximation of the logarithm of a value relative to some contextually understood reference value, usually 10, interpreted as the base of the logarithm and the representative of values of magnitude one. Logarithmic d ...

* Orders of magnitude (data)
* RKM code
The RKM code, also referred to as "letter and numeral code for resistance and capacitance values and tolerances", "letter and digit code for resistance and capacitance values and tolerances", or informally as "R notation" is a notation to speci ...

* SI base unit
The SI base units are the standard units of measurement defined by the International System of Units (SI) for the seven base quantities of what is now known as the International System of Quantities: they are notably a basic set from which al ...

* Unified Code for Units of Measure The Unified Code for Units of Measure (UCUM) is a system of codes for unambiguously representing measurement units. Its primary purpose is machine-to-machine communication rather than communication between humans.
The code set includes all units de ...

Footnotes

References

External links

International Bureau of Weights and Measures

(BIPM)

SI prefixes at BIPM

* ttp://physics.nist.gov/cuu/Units/binary.html US NIST ''Definitions of the SI units: The binary prefixes'' {{Portal bar, Physics Numeral systems