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 caesium hyperfine frequency Cs; the speed of light in vacuum c; the Planck constant h; the elementary charge e; the Boltzmann constant k; the Avogadro constant NA; and the luminous efficacy of a defined visible radiation Kcd.

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:

Defining constant	Symbol	Numerical value	Unit
hyperfine transition frequency of Cs	Cs	9 192 631 770	Hz
speed of light in vacuum	c	299 792 458	m s–1
Planck constant	h	6.626 070 15 x 10–34	J s
elementary charge	e	1.602 176 634 x 10–19	C
Boltzmann constant	k	1.380 649 x 10–23	J K–1
Avogadro constant	NA	6.022 140 76 x 1023	mol–1
luminous efficacy	Kcd	683	lm W–1



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.



The International System of Units, the SI, is the system of units in which the unperturbed ground state hyperfine transition frequency of the caesium 133 atom Cs is 9 192 631 770 Hz, the speed of light in vacuum c is 299 792 458 m/s, the Planck constant h is 6.626 070 15 x 10–34 J s, the elementary charge e is 1.602 176 634 x 10–19 C, the Boltzmann constant k is 1.380 649 x 10–23 J/K, the Avogadro constant NA is 6.022 140 76 x 1023 mol–1, the luminous efficacy of monochromatic radiation of frequency 540 x 1012 Hz, Kcd, is 683 lm/W.

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 m² s^–2, C = A s, lm = cd m² m^–2 = cd sr, and W = kg m² 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:

Base quantity			Base unit
Name	Typical symbol		Name	Symbol
time	t		second	s
length	l, x, r, etc.		metre	m
mass	m		kilogram	kg
electric current	I, i		ampere	A
thermodynamic temperature	T		kelvin	K
amount of substance	n		mole	mol
luminous intensity	Iv		candela	cd

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 explicit-constant 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.

Resolution 1 of the CGPM (2018): On the revision of the International System of Units (SI)

Connect to the BIPM's YouTube channel to watch a recording of the live open session of the 26th General Conference on Weights and Measures which took place on 16 November 2018: https://www.youtube.com/thebipm

FAQs about the revision of the SI, that came into force on 20 May 2019 Information for users about the revision of the SI SI roadmap (updated 2018) Resolution 1 of the CGPM (2014): On the future revision of the International System of Units, the SI Resolution 1 of the CGPM (2011): On the possible future revision of the International System of Units, the SI

Promotional documents and information about the revision of the SI

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.