# Resolution 1 of the 26th CGPM

## On the revision of the International System of Units (SI)

The General Conference on Weights and Measures (CGPM), at its 26th meeting,

**considering**

- the essential requirement for an International System of Units (SI) that is uniform and accessible world-wide for international trade, high-technology manufacturing, human health and safety, protection of the environment, global climate studies and the basic science that underpins all these,
- that the SI units must be stable in the long term, internally self consistent and practically realizable being based on the present theoretical description of nature at the highest level,
- that a revision of the SI to meet these requirements was proposed in Resolution 1 adopted unanimously by the CGPM at its 24th meeting (2011) that laid out in detail a new way of defining the SI based on a set of seven defining constants, drawn from the fundamental constants of physics and other constants of nature, from which the definitions of the seven base units are deduced,
- that the conditions set by the CGPM at its 24th meeting (2011), confirmed at its 25th meeting (2014), before such a revised SI could be adopted have now been met,

**decides** that, effective from 20 May 2019, 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 Δν
_{Cs}is9 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 × 10^{–34}J s, - the elementary charge
*e*is 1.602 176 634 × 10^{–19}C, - the Boltzmann constant
*k*is 1.380 649 × 10^{–23}J/K, - the Avogadro constant
*N*_{A}is 6.022 140 76 × 10^{23}mol^{–1}, - the luminous efficacy of monochromatic radiation of frequency 540 × 10
^{12}Hz,*K*_{cd}, 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 ^{–1}^{2} s^{–2}^{2} m^{–2} = ^{2} s^{–3}

**notes** the consequences as set out in Resolution 1 adopted by the CGPM at its 24th meeting (2011) in respect of the base units of the SI and confirms these in the following Appendices to this Resolution, which have the same force as the Resolution itself,

**invites** the International Committee for Weights and Measures (CIPM) to produce a new edition of its Brochure entitled "*The International System of Units*" in which a full description of the revised SI will be given.

**Appendix 1. Abrogation of former definitions of the base units**

It follows from the new definition of the SI described above that, effective from 20 May 2019:

- the definition of the second in force since 1967/68 (13th meeting of the CGPM, Resolution 1) is abrogated,
- the definition of the metre in force since 1983 (17th meeting of the CGPM, Resolution 1) is abrogated,
- the definition of the kilogram in force since 1889 (1st meeting of the CGPM, 1889, 3rd meeting of the CGPM, 1901) based upon the mass of the international prototype of the kilogram is abrogated,
- the definition of the ampere in force since 1948 (9th meeting of the CGPM) based upon the definition proposed by the CIPM (1946, Resolution 2) is abrogated,
- the definition of the kelvin in force since 1967/68 (13th meeting of the CGPM, Resolution 4) is abrogated,
- the definition of the mole in force since 1971 (14th meeting of the CGPM, Resolution 3) is abrogated,
- the definition of the candela in force since 1979 (16th meeting of the CGPM, Resolution 3) is abrogated,
- the decision to adopt the conventional values of the Josephson constant
*K*_{J-90}and of the von Klitzing constant*R*_{K-90}taken by the CIPM (1988, Recommendations 1 and 2) at the request of the CGPM (18th meeting of the CGPM, 1987, Resolution 6) for the establishment of representations of the volt and the ohm using the Josephson and quantum Hall effects, respectively, is abrogated.

**Appendix 2. Status of constants previously used in the former definitions**

It follows from the new definition of the SI described above, and from the recommended values of the 2017 special adjustment of the Committee on Data for Science and Technology (CODATA) on which the values of the defining constants are based, that effective from 20 May 2019:

- the mass of the international prototype of the kilogram
*m*(K) is equal to 1 kg within a relative standard uncertainty equal to that of the recommended value of*h*at the time this Resolution was adopted, namely 1.0 × 10^{–8}and that in the future its value will be determined experimentally, - the vacuum magnetic permeability
*μ*_{0}is equal to4π × 10 within a relative standard uncertainty equal to that of the recommended value of the fine-structure constant^{–7}H m^{–1}*α*at the time this Resolution was adopted, namely 2.3 × 10^{–10}and that in the future its value will be determined experimentally, - the thermodynamic temperature of the triple point of water T
_{TPW}is equal to 273.16 K within a relative standard uncertainty closely equal to that of the recommended value of*k*at the time this Resolution was adopted, namely 3.7 × 10^{–7}, and that in the future its value will be determined experimentally, - the molar mass of carbon 12,
*M*(^{12}C), is equal to 0.012 kg mol^{–1}within a relative standard uncertainty equal to that of the recommended value of*N*_{A}*h*at the time this Resolution was adopted, namely4.5 × 10 and that in the future its value will be determined experimentally.^{–10},

**Appendix 3. The base units of the SI**

Starting from the new definition of the SI described above in terms of fixed numerical values of the defining constants, definitions of each of the seven base units are deduced by taking, as appropriate, one or more of these defining constants to give the following set of definitions, effective from 20 May 2019:

- The second, symbol s, is the SI unit of time. It is defined by taking the fixed numerical value of the caesium frequency Δν
_{Cs}, the unperturbed ground-state hyperfine transition frequency of the caesium 133 atom, to be9 192 631 770 when expressed in the unit Hz, which is equal to s^{–1}. - The 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, where the second is defined in terms of Δν_{Cs}. - The 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 6.626 070 15 × 10^{–34}when expressed in the unit J s, which is equal tokg m where the metre and the second are defined in terms of^{2}s^{–1},*c*and Δν_{Cs}. - The 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 1.602 176 634 × 10^{–19}when expressed in the unit C, which is equal toA s, where the second is defined in terms of Δν_{Cs}. - The 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 1.380 649 × 10^{–23}when expressed in the unitJ K which is equal to^{–1},kg m where the kilogram, metre and second are defined in terms of^{2}s^{–2}K^{–1},*h*,*c*and Δν_{Cs}. - The mole, symbol mol, is the SI unit of amount of substance. One mole contains exactly
6.022 140 76 × 10 elementary entities. This number is the fixed numerical value of the Avogadro constant,^{23}*N*_{A}, when expressed in the unit mol^{–1}and is called the Avogadro number.

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 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 540 × 10
^{12}Hz, K_{cd}, to be 683 when expressed in the unit lm W^{–1}, which is equal tocd sr W or^{–1},cd sr kg where the kilogram, metre and second are defined in terms of^{–1}m^{–2}s^{3},*h*,*c*and Δν_{Cs}.