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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)

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Changes over time for: Division 1.1.

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[X1 [F11.1. 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 .] ]

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