– the intergovernmental organization through which Member States act together on matters related to measurement science and measurement standards. 


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. In a landmark decision, Member States voted on 16 November 2018 to revise the SI, changing the world's definition of the kilogram, the ampere, the kelvin and the mole. This decision, made at the 26th meeting of the General Conference on Weights and Measures (CGPM), means that from 20 May 2019 all SI units are defined in terms of constants that describe the natural world. This will assure the future stability of the SI and open the opportunity for the use of new technologies, including quantum technologies, to implement the definitions. The seven defining constants of the SI are:
The SI was previously defined in terms of seven base units and derived units defined as products of powers of the base units. The seven base units were chosen for historical reasons, and were, by convention, regarded as dimensionally independent: the metre, the kilogram, the second, the ampere, the kelvin, the mole, and the candela. This role for the base units continues in the present SI even though the SI itself is now defined in terms of the defining constants above.
The definition of the SI units is established in terms of a set of seven defining constants. The complete system of units can be derived from the fixed values of these defining constants, expressed in the units of the SI. These seven defining constants are the most fundamental feature of the definition of the entire system of units. The seven defining constants of the SI and the seven corresponding units they define:
These particular constants were chosen after having been identified as being the best choice, taking into account the previous definition of the SI, which was based on seven base units, and progress in science. The definitions below specify the exact numerical value of each constant when its value is expressed in the corresponding SI unit. By fixing the exact numerical value the unit becomes defined, since the product of the numerical value and the unit has to equal the value of the constant, which is postulated to be invariant. The seven constants are chosen in such a way that any unit of the SI can be written either through a defining constant itself or through products or quotients of defining constants.
where the hertz, joule, coulomb, lumen, and watt, with unit symbols Hz, J, C, lm, and W, respectively, are related to the units second, metre, kilogram, ampere, kelvin, mole, and candela, with unit symbols s, m, kg, A, K, mol, and cd, respectively, according to Hz = s^{–1}, J = kg m^{2} s^{–2}, C = A s, lm = cd m^{2} m^{–2} = cd sr, and W = kg m^{2} s^{–3}. The seven constants are chosen in such a way that any unit of the SI can be written either through a defining constant itself or through products or quotients of defining constants. The numerical values of the seven defining constants have no uncertainty. The SI base units:
DefinitionsStarting from the definition of the SI in terms of fixed numerical values of the defining constants, definitions of each of the seven base units are deduced by using, as appropriate, one or more of these defining constants to give the following set of definitions: The secondThe second, symbol s, is the SI unit of time. It is defined by taking the fixed numerical value of the caesium frequency _{Cs}, the unperturbed groundstate hyperfine transition frequency of the caesium133 atom, to be
The metreThe metre, symbol m, is the SI unit of length. It is defined by taking the fixed numerical value of the speed of light in vacuum c to be 299 792 458 when expressed in the unit m s^{–1}, where the second is defined in terms of the caesium frequency _{Cs}.
The kilogramThe kilogram, symbol kg, is the SI unit of mass. It is defined by taking the fixed numerical value of the Planck constant h to be
The ampereThe ampere, symbol A, is the SI unit of electric current. It is defined by taking the fixed numerical value of the elementary charge e to be
The kelvinThe kelvin, symbol K, is the SI unit of thermodynamic temperature. It is defined by taking the fixed numerical value of the Boltzmann constant k to be
The moleThe mole, symbol mol, is the SI unit of amount of substance. One mole contains exactly The amount of substance, symbol n, of a system is a measure of the number of specified elementary entities. An elementary entity may be an atom, a molecule, an ion, an electron, any other particle or specified group of particles.
The candelaThe candela, symbol cd, is the SI unit of luminous intensity in a given direction. It is defined by taking the fixed numerical value of the luminous efficacy of monochromatic radiation of frequency
All other SI units can be derived from these, by multiplying together different powers of the base units. In the 2018 revision of the SI, the definitions of four of the SI base units – the kilogram, the ampere, the kelvin and the mole – were changed. Their new definitions are based on fixed numerical values of the Planck constant (h), the elementary charge (e), the Boltzmann constant (k), and the Avogadro constant (N_{A}), respectively. Further, the definitions of all seven base units of the SI are now uniformly expressed using the explicitconstant formulation. Specific mises en pratique have been drawn up to explain the realization of the definitions of each of the base units in a practical way. The new definitions came into force on 20 May 2019.
Appendix 2 of the SI Brochure Thee mises en pratique are prepared by the relevant Consultative Committees, and, after approval by the CIPM, are then published in electronic form here on the BIPM website, where they may be revised more frequently than if they were printed in the SI Brochure.
Decimal multiples and submultiples of SI units can be written using the SI prefixes listed in the table below:
For full details please refer to Chapter 3 of the SI Brochure. 


