SI Unit definitions
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Current definitions (2019) of each of the SI base units are set
out below, following refinements implemented on 20 May 2019 which linked all of
the definitions to fundamental constants of nature, see here.
Metre: The metre
is the SI unit of length. It is the distance travelled by light in a vacuum
during a time interval of 1/299792458 of a second.
Kilogram: The
kilogram is the SI unit of mass. It is defined by setting Plank’s constant to exactly
6.62607015 x 10-34 joule second. It used to be defined (immediately
prior to 20 May 2019) as the mass of the international prototype of the
kilogram.
Second: The second
is the SI unit of time. It is the duration of 9192631770 periods of the
radiation corresponding to the transition between the two hyperfine levels of
the ground state of the cesium-133 atom.
Ampere: The ampere
is the SI unit of electrical current. It is defined by setting the elementary
charge e to be exactly 1.602176634 x 10-19 coulomb. It used
to be defined (immediately prior to 20 May 2019) as the constant current which,
if maintained in two straight parallel conductors of infinite length, of
negligible circular cross section, and placed 1 meter apart in vacuum, would
produce between these conductors a force equal to 2 x 10−7
newtons per meter of length.
Kelvin: The kelvin
is the SI unit of thermodynamic temperature. It is defined by setting the
Boltzmann constant 
to be exactly 1.380649 x 10-23
joule per kelvin. It used to be defined (immediately prior to 20 May 2019) as
the fraction 1/273.16 of the thermodynamic temperature of the triple point of
water.
Mole: The mole is
the SI unit of amount of atomic substance. It is defined by setting the Avogadro
constant 
to
be exactly 6.02214076 x 1023 reciprocal mole. It used to be defined (immediately
prior to 20 May 2019) as the amount of which contains as many elementary
entities as there are atoms in 0.012 kilogram of carbon 12. When the mole is used,
the elementary entities must be specified and may be atoms, molecules, ions,
electrons, other particles, or specified groups of such particles.
Candela: The
candela is SI unit of luminous intensity. It is the luminous intensity, in a
given direction, of a source that emits monochromatic radiation of frequency
540 x 1012 hertz and that has a radiant intensity in that direction
of (1/683) watt per steradian.
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