A metric prefix is a unit prefix that precedes a basic unit of measure
to indicate a multiple or fraction of the unit. While all metric
prefixes in common use today are decadic, historically there have been
a number of binary metric prefixes as well. Each prefix has a
unique symbol that is prepended to the 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.
1 List of SI prefixes 2 Application to units of measurement
2.1 Metric units
2.1.1 Mass 2.1.2 Volume 2.1.3 Length 2.1.4 Time and angles 2.1.5 Temperature 2.1.6 Energy
2.2 Non-metric units
3.1 Pronunciation 3.2 Typesetting
4 Non-standard prefixes
4.1 Obsolete metric prefixes 4.2 Double prefixes 4.3 "Hella" prefix proposal 4.4 X, W and V
5 Similar symbols and abbreviations
5.1 Binary prefixes
6 See also 7 Notes 8 References 9 External links
List of SI prefixes The BIPM specifies twenty prefixes for the International System of Units (SI).
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Prefix Base 1000 Base 10 Decimal English word Adoption[nb 1]
Name Symbol Short scale Long scale
yotta Y 10008 1024 1000000000000000000000000 septillion quadrillion 1991
zetta Z 10007 1021 1000000000000000000000 sextillion trilliard 1991
exa E 10006 1018 1000000000000000000 quintillion trillion 1975
peta P 10005 1015 1000000000000000 quadrillion billiard 1975
tera T 10004 1012 1000000000000 trillion billion 1960
giga G 10003 109 1000000000 billion milliard 1960
mega M 10002 106 1000000 million 1873
kilo k 10001 103 1000 thousand 1795
hecto h 10002/3 102 100 hundred 1795
deca da 10001/3 101 10 ten 1795
10000 100 1 one –
deci d 1000−1/3 10−1 0.1 tenth 1795
centi c 1000−2/3 10−2 0.01 hundredth 1795
milli m 1000−1 10−3 0.001 thousandth 1795
micro µ 1000−2 10−6 0.000001 millionth 1873
nano n 1000−3 10−9 0.000000001 billionth milliardth 1960
pico p 1000−4 10−12 0.000000000001 trillionth billionth 1960
femto f 1000−5 10−15 0.000000000000001 quadrillionth billiardth 1964
atto a 1000−6 10−18 0.000000000000000001 quintillionth trillionth 1964
zepto z 1000−7 10−21 0.000000000000000000001 sextillionth trilliardth 1991
yocto y 1000−8 10−24 0.000000000000000000000001 septillionth quadrillionth 1991
^ Prefixes adopted before 1960 already existed before SI. 1873 was the introduction of the CGS system.
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. Where
Greek letters are unavailable, the symbol for micro 'µ' is commonly
replaced by 'u'.
Prefixes corresponding to an integer power of one thousand are
generally preferred. Hence 100 m is preferred over 1 hm
(hectometre) or 10 dam (decametres). The prefixes hecto, deca,
deci, and centi are commonly used for everyday purposes, and the
centimetre (cm) is especially common. However, 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".
Prefixes may not be used in combination. This also applies to mass,
for which the
SI base unit
6997500000000000000♠5 mV × 6997500000000000000♠5 mA = 6997500000000000000♠5×10−3 V × 6997500000000000000♠5×10−3 A = 6995249999999999999♠25×10−6 V⋅A = 6995249999999999999♠25 μW 6997500000000000000♠5.00 mV + 6995099999999999999♠10 μV = 6997500000000000000♠5.00 mV + 6995100000000000000♠0.01 mV = 6997501000000000000♠5.01 mV
When units occur in exponentiation, for example, in square and cubic forms, the multiplication prefix must be considered part of the unit, and thus included in the exponentiation.
1 km2 means one square kilometre, or the area of a square of 7003100000000000000♠1000 m by 7003100000000000000♠1000 m and not 7003100000000000000♠1000 square metres. 2 Mm3 means two cubic megametres, or the volume of two cubes of 7006100000000000000♠1000000 m by 7006100000000000000♠1000000 m by 7006100000000000000♠1000000 m or 7018200000000000000♠2×1018 m3, and not 7006200000000000000♠2000000 cubic metres (7006200000000000000♠2×106 m3).
6998500000000000000♠5 cm = 6998500000000000000♠5×10−2 m = 5 × 0.01 m = 0.05 m 7006900000000000000♠9 km2 = 9 × (103 m)2 = 9 × (103)2 × m2 = 7006900000000000000♠9×106 m2 = 9 × 7006100000000000000♠1000000 m2 = 7006900000000000000♠9000000 m2 3 MW = 7006300000000000000♠3×106 W = 3 × 7006100000000000000♠1000000 W = 7006300000000000000♠3000000 W
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.,
millidynes and milligauss). Metric prefixes may also be used with
The choice of prefixes with a given unit is usually dictated by
convenience of use. Unit prefixes for amounts that are much larger or
smaller than those actually encountered are seldom used.
In use, the kilogram, gram, milligram, microgram, and smaller are
fairly common. However, megagram (and gigagram, teragram, etc.) are
rarely used; tonnes (and kilotonnes, megatonnes, etc. – although
these units generally are not used as a measure of mass per se, but
rather TNT energy equivalent of a mass) or scientific notation are
used instead. Megagram is occasionally used to disambiguate the metric
tonne from the various non-metric tons. An exception is pollution
emission rates, which are typically on the order of Tg/yr. Sometimes,
only one element or compound is denoted for an emission, such as
Tg C/yr or Tg N/yr.
Alone among SI units, the base unit of mass, the kilogram, already
includes a prefix. The prefixes consequently do not indicate
corresponding multipliers of the base unit in the case of mass; for
example, a megagram is 7003100000000000000♠1×103 kg, whereas
mega- indicates a multiplier of 7006100000000000000♠106.
The litre (equal to a cubic decimetre), millilitre (equal to a cubic
centimetre), microlitre, and smaller are common. In Europe, the
centilitre is often used for packaged products (such as wine) and the
decilitre less frequently. (The latter two items include prefixes
corresponding to an exponent that is not divisible by three.)
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.
The kilometre, metre, centimetre, millimetre, and smaller are common.
(However, the decimetre is rarely used.) The micrometre is often
referred to by the non-SI term micron. In some fields, such as
chemistry, the ångström (equal to 0.1 nm) historically competed
with 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, non-metric units are used, such
as astronomical units, light years, and parsecs; the astronomical unit
is mentioned in the SI standards as an accepted non-SI unit.
Time and angles
The second, millisecond, microsecond, and shorter are common. The
kilosecond and megasecond also have some use, though for these and
longer times one usually uses either scientific notation or minutes,
hours, and so on.
Official policies about the use of these prefixes vary slightly
between the Bureau International des Poids et Mesures (BIPM) and the
National Institute of Standards and Technology
Distance marker on the Rhine: 36 (XXXVI) myriametres from Basel. The
stated distance is 360 km; the decimal mark in
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 myria- (sometimes also written as myrio-) (ten thousand) as
well as the binary prefixes double- and demi-, denoting a factor of 2
and 1/2 (one half), respectively, were parts of the original metric
system adopted by France in 1795. These were not retained when the
SI prefixes were internationally adopted by the 11th CGPM conference
Other metric prefixes used historically include hebdo- (107) and
Double prefixes have been used in the past, such as micromillimetres
or "millimicrons" (now nanometres), micromicrofarads (now picofarads),
kilomegatons (now gigatons), hectokilometres (now 100 kilometres) and
the derived adjective hectokilometric (typically used for qualifying
the fuel consumption measures). These were disallowed with the
introduction of the SI.
Other obsolete double prefixes included "decimilli-" (10−4), which
was contracted to "dimi-" and standardized in France up to 1961.
"Hella" prefix proposal
Engineering notation Indian Numbering System International vocabulary of metrology ISO/IEC 80000 List of numbers in various languages (for comparison/etymology) Names of large numbers Names of small numbers Number names Numeral prefix Order of magnitude Letter and digit code SI base unit
^ The names and symbols of the binary prefixes proposed by the IEC include
kibi (Ki) = 210 = 7003102400000000000♠1024 mebi (Mi) = 220 = 7003102400000000000♠10242 = 7006104857600000000♠1048576 gibi (Gi) = 230 = 7003102400000000000♠10243 = 7009107374182400000♠1073741824
References This article is based on material taken from the Free On-line Dictionary of Computing prior to 1 November 2008 and incorporated under the "relicensing" terms of the GFDL, version 1.3 or later.
^ a b "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" (in French).
histoire.du.metre.free.fr. Retrieved 12 October 2015.
^ "Four Resolutions". Bipm.org. Retrieved 1 March 2012.
^ Suplee, Curt (2009-07-02). "
Bureau International des Poids et Mesures (BIPM) SI prefixes at BIPM US NIST Definitions of the SI units: The twenty SI prefixes US NIST Definitions of the SI units: The binary prefixes
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International System of Units
ampere candela kelvin kilogram metre mole second
Derived units with special names
becquerel coulomb degree Celsius farad gray henry hertz joule katal lumen lux newton ohm pascal radian siemens sievert steradian tesla volt watt weber
Other accepted units
astronomical unit bar dalton day decibel degree of arc electronvolt hectare hour litre minute minute of arc neper second of arc tonne atomic units natural units
Conversion of units Metric prefixes Proposed redefinitions Systems of measurement
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Orders of magnitude
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