The International System of Units (abbreviated SI from French Système international d'unités) is the modern form of the metric system and is a system of units of measurement devised around seven base units and the convenience of the number ten. It is the world's most widely used system of measurement, both in everyday commerce and in science.
SI consists of a set of units together with a set of prefixes. The units are divided into two classes—base units and derived units. There are seven base units, each representing a different physical quantity. The prefixes may be combined with the base units and derived units to describe the number of those units. For example, 3×1015 newton = 3 petanewton, combining the prefix peta-, for quadrillion, with the derived unit, newton.
SI is an abbreviation of Système International (d'Unités) and is a standard metric system of units adopted for official scientific use. For more information, see Wikipedia's article on SI.
Note: this page is simply a collection of links to the names of SI units and their prefixes and multiples. For further information, see the Wikipedia articles SI base unit and SI derived unit.
Prefix | Symbol | 1000m | 10n | Decimal | Short scale | Long scale | Since[n 1] |
---|---|---|---|---|---|---|---|
quetta | Q | 100010 | 1030 | 1000000000000000000000000000000 | Nonillion | Quintillion | 2022 |
ronna | R | 10009 | 1027 | 1000000000000000000000000000 | Octillion | Quadrillard | 2022 |
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 | 1960 | |
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 | 1960 | |
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 |
ronto | r | 1000−9 | 10−27 | 0.000000000000000000000000001 | Octillionth | Quadrillardth | 2022 |
quecto | q | 1000−10 | 10−30 | 0.000000000000000000000000000001 | Nonillionth | Quintillionth | 2022 |
Notes:
The following prefixes are not part of SI. They were adopted by the IEC to express binary multiples.
prefix | sym | multiplier |
---|---|---|
yobi- | Yi | 280 |
zebi- | Zi | 270 |
exbi- | Ei | 260 |
pebi- | Pi | 250 |
tebi- | Ti | 240 |
gibi- | Gi | 230 |
mebi- | Mi | 220 |
kibi- | Ki | 210 |
These prefixes have been used informally at times, but were never part of SI.
prefix | multiplier | equivalent |
---|---|---|
hectokilo- | 105 | |
myria- | 104 | |
millimilli- | 10–6 | micro- |
millimicro- | 10–9 | nano- |
micromicro- | 10–12 | pico- |
(Sources: ; Oxford English Dictionary, 2nd ed.)
Y | Z | E | P | T | G | M | k | h | da | Unit | d | c | m | μ | n | p | f | a | z | y |
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
MA | kA | A | mA | μA | nA | |||||||||||||||
PBq | TBq | GBq | Bq | |||||||||||||||||
C | ||||||||||||||||||||
°C | ||||||||||||||||||||
cd | ||||||||||||||||||||
F | mF | μF | nF | pF | ||||||||||||||||
t, Mg | kg | hg | dag | g | dg | cg | mg | μg | ||||||||||||
Gy | ||||||||||||||||||||
H | mH | μH | nH | |||||||||||||||||
PHz | THz | GHz | MHz | kHz | Hz | |||||||||||||||
MJ | kJ | J | ||||||||||||||||||
K | ||||||||||||||||||||
kat | ||||||||||||||||||||
lm | ||||||||||||||||||||
klx | lx | |||||||||||||||||||
km | hm | dam | m | dm | cm | mm | μm | nm | pm | fm | ||||||||||
mol | mmol | μmol | nmol | pmol | fmol | ymol | ||||||||||||||
MN | kN | N | ||||||||||||||||||
GΩ | MΩ | kΩ | Ω | mΩ | ||||||||||||||||
GPa | MPa | kPa | hPa | Pa | ||||||||||||||||
rad | mrad | |||||||||||||||||||
S | ||||||||||||||||||||
s | ms | μs | ns | ps | fs | as | ||||||||||||||
sr | ||||||||||||||||||||
Sv | mSv | µSv | ||||||||||||||||||
T | mT | µT | nT | |||||||||||||||||
MV | kV | V | mV | μV | ||||||||||||||||
EW | PW | TW | GW | MW | kW | W | mW | μW | ||||||||||||
kWb | Wb | mWb | μWb |
Name | Symbol | Measure | Definition | Historical origin / justification |
---|---|---|---|---|
second | s | time | "The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom." 13th CGPM (1967/68, Resolution 1; CR, 103) "This definition refers to a caesium atom at rest at a temperature of 0 K." (Added by CIPM in 1997) |
The day is divided in 24 hours, each hour is 60 minutes, and each minute is 60 seconds. 24×60×60 = 86,400. So a second is (1⁄86,400) day. Cesium-133 is the only stable isotope of cesium. |
metre | m | length | "The metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second." 17th CGPM (1983, Resolution 1, CR, 97) |
1⁄107 of the distance from the Earth's equator to the North Pole measured on the circumference through Paris. |
kilogram | kg | mass | "The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram." 3rd CGPM (1901, CR, 70) |
The mass of one litre of water. A litre is (1/1,000) cubic metre. Note that this unit has the kilo- prefix and is used by convention and for historical reasons, rather than the base gram. |
ampere | A | electric current | "The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 metre apart in vacuum, would produce between these conductors a force equal to 2 × 10−7 newton per metre of length." 9th CGPM (1948) |
The original "International Ampere" was defined electrochemically as the current required to deposit 1.118 milligrams of silver per second from a solution of silver nitrate. Compared to the SI ampere, the difference is 0.015%. |
kelvin | K | thermodynamic temperature | "The kelvin, unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water." 13th CGPM (1967/68, Resolution 4; CR, 104) "This definition refers to water having the isotopic composition defined exactly by the following amount of substance ratios: 0.000 155 76 mole of 2H per mole of 1H, 0.000 379 9 mole of 17O per mole of 16O, and 0.002 005 2 mole of 18O per mole of 16O." (Added by CIPM in 2005) |
The Celsius scale: the Kelvin scale uses the degree Celsius for its unit increment, but is a thermodynamic scale (0 K is absolute zero). |
mole | mol | amount of substance | "1. The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12; its symbol is “mol.”
2. When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles." |
Atomic weight or molecular weight divided by the molar mass constant, 1 g/mol. |
candela | cd | luminous intensity | "The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 × 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian." 16th CGPM (1979, Resolution 3; CR, 100) |
The candlepower, which is based on the light emitted from a burning candle of standard properties. |
Base units can be combined to derive units of measurement for other quantities. In addition to the two dimensionless derived units radian (rad) and steradian (sr), 20 other derived units have special names.
Name | Symbol | Quantity | Expression in terms of other units | Expression in terms of SI base units |
---|---|---|---|---|
hertz | Hz | frequency | 1/s | s−1 |
radian | rad | angle | m/m | dimensionless |
steradian | sr | solid angle | m2/m2 | dimensionless |
newton | N | force, weight | kg·m/s2 | kg·m·s−2 |
pascal | Pa | pressure, stress | N/m2 | kg·m−1·s−2 |
joule | J | energy, work, heat | N·m = C·V = W·s | kg·m2·s−2 |
watt | W | power, radiant flux | J/s = V·A | kg·m2·s−3 |
coulomb | C | electric charge or quantity of electricity | s·A | s·A |
volt | V | voltage, potential difference, electromotive force | W/A = J/C | kg·m2·s−3·A−1 |
farad | F | capacitance | C/V | kg−1·m−2·s4·A2 |
ohm | Ω | electrical resistance, impedance, reactance | V/A | kg·m2·s−3·A−2 |
siemens | S | electrical conductance | 1/Ω = A/V | kg−1·m−2·s3·A2 |
weber | Wb | magnetic flux | J/A | kg·m2·s−2·A−1 |
tesla | T | magnetic field strength, magnetic flux density | V·s/m2 = Wb/m2 = N/(A·m) | kg·s−2·A−1 |
henry | H | inductance | V·s/A = Wb/A | kg·m2·s−2·A−2 |
degree Celsius | °C | temperature relative to 273.15 K | K | K |
lumen | lm | luminous flux | cd·sr | cd |
lux | lx | illuminance | lm/m2 | m−2·cd |
becquerel | Bq | radioactivity (decays per unit time) | 1/s | s−1 |
gray | Gy | absorbed dose (of ionizing radiation) | J/kg | m2·s−2 |
sievert | Sv | equivalent dose (of ionizing radiation) | J/kg | m2·s−2 |
katal | kat | catalytic activity | mol/s | s−1·mol |
Other common units, such as the litre, are not SI units, but are accepted for use with SI.
Notes:
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In French, accents appear in the following prefixes and unit-names:
In German, common nouns (including the names of SI units) are capitalized. Some spelling variations also appear:
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– CODATA reportIn the International System of Units (SI) (BIPM, 2006), the definition of the metre fixes the speed of light in vacuum c0, the definition of the ampere fixes the magnetic constant (also called the permeability of vacuum) μ0, and the definition of the mole fixes the molar mass of the carbon-12 atom M(12C) to have the exact values given in the table . Since the electric constant (also called the permittivity of vacuum) is related to μ0 by ε0 = 1/μ0c02, it too is known exactly.