Council Directive of 20 December 1979 on the approximation of the laws of the Member States relating to units of measurement and on the repeal of Directive 71/354/EEC (80/181/EEC)

[X1 [F1 1.1. SI base units U.K.

Quantity	Unit
	Name	Symbol
Time	second	s
Length	metre	m
Mass	kilogram	kg
Electric current	ampere	A
Thermodynamic temperature	kelvin	K
Amount of substance	mole	mol
Luminous intensity	candela	cd

Definitions of SI base units:

• Unit of time

  The second, symbol s, is the SI unit of time. It is defined by taking the fixed numerical value of the caesium frequency Δ ν _Cs , the unperturbed ground-state hyperfine transition frequency of the caesium 133 atom, to be 9 192 631 770 when expressed in the unit Hz, which is equal to s ^–1 .

• Unit of length

  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 Δ ν _Cs .

• Unit of mass

  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 to kg m ² s ^–1 , where the metre and the second are defined in terms of c and Δ ν _Cs .

• Unit of electric current

  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 to A s, where the second is defined in terms of Δ ν _Cs .

• Unit of thermodynamic temperature

  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 unit J K ^–1 , which is equal to kg m ² s ^–2 K ^–1 , where the kilogram, metre and second are defined in terms of h , c and Δ ν _Cs .

• Unit of amount of substance

  The mole, symbol mol, is the SI unit of amount of substance. One mole contains exactly 6,022 140 76 × 10 ²³ elementary entities. This number is the fixed numerical value of the Avogadro constant, 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.

• Unit of luminous intensity

  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 to cd sr W ^–1 , or cd sr kg ^–1 m ^–2 s ³ , where the kilogram, metre and second are defined in terms of h , c and Δ ν _Cs .

1.1.1. Special name and symbol of the SI derived unit of temperature for expressing Celsius temperature U.K.

Quantity	Unit
	Name	Symbol
Celsius temperature	degree Celsius	°C

Celsius temperature t is defined as the difference t = T – T ₀ between the two thermodynamic temperatures T and T ₀ where T ₀   = 273,15 K. An interval or difference of temperature may be expressed either in kelvins or in degrees Celsius. The unit ‘ degree Celsius ’ is equal to the unit ‘ kelvin ’ .] ]

Editorial Information

X1 Substituted by Corrigendum to Council Directive 80/181/EEC of 20 December 1979 on the aproximation of the laws of the Member States relating to units of measurement and on the repeal of Directive 71/354/EEC (Official Journal of the European Communities No L 39 of 15 February 1980).

Textual Amendments

F1 Substituted by Commission Directive (EU) 2019/1258 of 23 July 2019 amending, for the purpose of its adaptation to technical progress, the Annex to Council Directive 80/181/EEC as regards the definitions of SI base units (Text with EEA relevance).