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For other uses, see SI (disambiguation).
The International System of Units, (symbol: SI) (for the French phrase Système International d'Unités), is the most widely used system of units. It is used for everyday commerce in virtually every country of the world except the United States. SI was selected from the existing Metre-Kilogram-Second system of units (MKS), with the addition of extra units, rather than the older Centimetre-Gram-Second system of units (CGS). SI is sometimes referred to as the metric system (especially in the United States, which has not widely adopted it, and the UK, where conversion is incomplete). However, not all metric units of measurement are accepted as SI units

There are seven base units and several derived units, together with a set of prefixeses. Non-SI units can be converted to SI units (or vice versa) according to the conversion of units.

Table of contents
1 Origin
2 Basis
3 SI writing style
4 Units
5 Spelling variations
6 Related topics
7 External links
8 Further reading


The units of the SI system are decided by international conferences organised by the Bureau International des Poids et Mesures (International Office of Weights and Measures). The SI system was first given its name in 1960, and last added to in 1971.


SI is built on seven SI base units, such as the kilogram, metre and second. These are used to define various SI derived units.

SI also defines a number of SI prefixes to be used with the units: these combine with any unit name to give subdivisions and multiples. For example, the prefix kilo denotes a multiple of a thousand, so the kilometre is 1 000 metres, the kilogram 1 000 grams, and so on. Note that a millionth of a kilogram is a milligram, not a microkilogram.

SI writing style

The system can legally be used in every country in the world, and in many countries its use is obligatory. Those countries that still give official recognition to non-SI units (e.g. the US and UK) define them in terms of SI units; for example, the inch is defined to be exactly 0.0254 metres. It was adopted by the 11th General Conference on Weights and Measures (CGPM) in 1960. (See weights and measures for a history of the development of units of measurement.)


Base Units

The following are the fundamental units from which all others are derived, they are dimensionally independent. The definitions stated below are widely accepted.

Name Unit Symbol Measure Of Definition
metre m Length The unit of length is equal to the length of the path travelled by light in a vacuum during the time interval of 1/299 792 458 of a second
kilogram kg Mass The unit of mass is equal to the mass of the international prototype kilogram (a platinum-iridium cylinder) kept at the Bureau International des Poids et Mesures (BIPM), Sèvres, Paris.
second s Time The unit of time is the duration of exactly 9 192 631 770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of caesium-133 atom.
ampere A Electrical Current The unit of electrical current is the constant current which, if maintained in two straight parallel conductors, of infinite length and negligible cross-section, placed 1 metre apart in a vacuum, would produce a force between these conductors equal to 2×10 −7 newton per metre of length.
kelvin K Absolute Temperature The unit of thermodynamic temperature (or absolute temperature) is the fraction 1/273.16 (exactly) of the thermodynamic temperature at the triple point of water.
mole mol Amount of substance The unit of amount of substance is the amount of substance which contains as many elementary entities as there are atoms in 0.012 kilogram of pure carbon-12. [elementary entities may be atoms, molecules, ions, electrons, or particles].
candela cd Luminous intensity The unit of luminous intensity 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.

Dimensionless derived units

The following SI units are derived from the base units and are dimensionless.

Name Unit Symbol Measure Of Definition
radian rad Angle The unit of angle is the angle subtended at the centre of a circle by an arc of the circumference equal in length to the radius of the circle. There are radians in a circle.
steradian sr Solid Angle The unit of solid angle is the solid angle subtended at the centre of a sphere of radius r by a portion of the surface of the sphere having an area r2. There are steradians in a sphere.

Derived units with special names

Base units can be put together to derive units of measurement for other quantities. Some have been given names.

Name Unit Symbol Measure of Expressed in base units
hertz Hz Frequency s-1
newton N Force kg m/s 2
joule J Energy N m = kg m2/s2
watt W Power J/s = kg m2/s3
pascal Pa Pressure N/m2 = kg/m s2
lumen lm Luminous flux cd sr
lux lx Illuminance cd sr/m2
coulomb C Electric Charge A s
volt V Electric Potential Difference J/C = kg m2 A-1 s-3
ohm Ω Electric resistance V/A = kg m2 A-2 s-3
farad F Electric capacitance A2 s4 kg-1 m-2 = -1 s
weber Wb Magnetic flux kg m2/s2 A
tesla T Magnetic flux density Wb/m2 = kg/s2 A
henry H Inductance kg m2 A-2 s-2 = s
siemens S Electric conductance -1 = kg-1 m-2 A2 s3
becquerel Bq Radioactivity (decays per unit time) s-1
gray Gy Absorbed dose (of ionising radiation) J/kg
sievert Sv Dose equivalent (of ionising radiation) J/kg

The unit of volume litre, abbreviated L or l and being equal to 0.001 m3, is not an SI unit but is "accepted for use with the International System."

Spelling variations

Several nations, notably the United States, typically use the spellings 'meter' and 'liter' instead of 'metre' and 'litre', in keeping with standard American English spelling (for example, Americans also use 'center' rather than 'centre'; see also American and British English differences). In addition, the official US spelling for the SI prefix 'deca' is 'deka' (again, a variation not recognized by the BIPM).

The US government has approved these spellings for official use, but the BIPM only recognizes the British English spellings as official names for the units. In scientific contexts only the abbreviations are used; since these are universally the same, the differences do not arise in practice in scientific use.

The unit 'gram' is also sometimes spelled 'gramme' in English-speaking countries, though that is an older spelling and is falling out of use.

Related topics

External links



Further reading