Metric units are
units based on the metre, gram or second and decimal (power of ten) multiples or sub-multiples of these. According to Schadow and McDonald,[1] metric units, in general, are those units "defined 'in the spirit' of the metric system, that emerged in late 18th century France and was rapidly adopted by scientists and engineers. Metric units are in general based on reproducible natural phenomena and are usually not part of a system of comparable units with different magnitudes, especially not if the ratios of these units are not powers of 10. Instead, metric units use multiplier prefixes that magnifies or diminishes the value of the unit by powers of ten."
The most widely used examples are the units of the
International System of Units (SI). By extension they include units of
electromagnetism from the
CGS and SI units systems, and other units for which use of
SI prefixes has become the norm. Other unit systems using metric units include:
The first group of metric units are those that are at present defined as units within the International System of Units (SI). In its most restrictive interpretation, this is what may be meant when the term metric unit is used.
The
gray (Gy) is equal to one joule per kilogram (1 J⋅kg−1).
The
sievert (Sv) is equal to one joule per kilogram (1 J⋅kg−1).
The
katal (kat) is equal to one mole per second (1 mol⋅s−1).
Furthermore, there are twenty-four metric prefixes that can be combined with any of these units except one (1) and kilogram (kg) to form further units of the SI. For mass, the same prefixes are applied to the gram (g) instead of the kilogram.
Non-SI metric units
There are several metric systems, most of which have become disused or are still used in only niche disciplines. Systems are listed with named units that are associated with them.
The
centimetre–gram–second system of units (CGS) is based on three base units: centimetre, gram and second. Its subsystems (
CGS-ESU,
CGS-EMU and
CGS-Gaussian) have different defining equations for their systems of quantities for defining electromagnetic quantities and hence the associated units, with CGS-Gaussian units being selected from each of the other two subsystems.
The CGS-to-SI correspondence of electromagnetic units as given was exact prior to the
2019 redefinition of the SI base units, until which the
magnetic constantμ0 was defined as 4π×10−7 N⋅A−2. As from the redefinition, μ0 has an inexactly known value when expressed in SI units, with the exactness of the electromagnetic unit correspondence given here being affected accordingly.
CGS nonelectromagnetic units
The
kayser (K) is a unit of
wavenumber equal to 1 cm−1 (100 m−1).
The
einstein (E) has two conflicting definitions. The original is a unit of
energy, equal to the energy in one
mole (1 mol) of photons. The second is a unit of
amount of
photons, equal to one
mole (1 mol) of photons.
The
rayleigh (R) is a unit of photon flux rate density equal to 1010 m−2⋅s−1 (104 mm−2⋅s−1).