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Measurement units: the SI

The recommended practical system of units of measurement is the International System of Units (Système International d'Unités, with the international abbreviation SI).

The SI is defined by the SI Brochure, which is published by the BIPM.

This SI consists of a set of base units, prefixes and derived units, as described in these pages:

  • The SI base units are a choice of seven well-defined units which by convention are regarded as dimensionally independent: the metre, the kilogram, the second, the ampere, the kelvin, the mole, and the candela.

  • Derived units are formed by combining the base units according to the algebraic relations linking the corresponding quantities. The names and symbols of some of the units thus formed can be replaced by special names and symbols which can themselves be used to form expressions and symbols of other derived units.

The SI is not static but evolves to match the world's increasingly demanding requirements for measurement. Currently much work is under way related to the intended future revision of the SI.

The SI base units are a choice of seven well-defined units which by convention are regarded as dimensionally independent:

metre, m

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.

kilogram, kg

The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram.

second, s

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.

ampere, A

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 m apart in vacuum, would produce between these conductors a force equal to 2 x 10–7 newton per metre of length.

kelvin, K

The kelvin, unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.

mole, mol
  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.
  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.
candela, cd

The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.

All other SI units can be derived from these, by multiplying together different powers of the base units.

Unique publications for international metrology:

Decimal multiples and submultiples of SI units can be written using the SI prefixes listed in the table below:

Factor Name Symbol Factor Name Symbol
101 deca da 10–1 deci d
102 hecto h 10–2 centi c
103 kilo k 10–3 milli m
106 mega M 10–6 micro µ
109 giga G 10–9 nano n
1012 tera T 10–12 pico p
1015 peta P 10–15 femto f
1018 exa E 10–18 atto a
1021 zetta Z 10–21 zepto z
1024 yotta Y 10–24 yocto y

For full details please refer to Chapter 3 of the SI Brochure.