The metric system is an internationally adopted
In its modern form, it consists of a set of
The metric system was designed to have properties that make it easy to use and widely applicable, including units based on the natural world, decimal ratios, prefixes for multiples and sub-multiples, and a structure of base and derived units. It is also a
The units of the metric system, originally taken from observable features of nature, are now realised by synthetic phenomena such as the microwave frequency of a caesium atomic clock which accurately measures seconds. One unit, the kilogram, remains defined in terms of a man-made artefact.
While there are numerous named derived units of the metric system, such as watt and lumen, other common quantities such as velocity and acceleration do not have their own unit, but are defined in terms of existing base and derived units such as metres per second for velocity.
Though other currently or formerly widespread systems of weights and measures continue to exist, such as the
The metric system is also extensible, and new base and derived units are defined as needed in fields such as radiology and chemistry. The most recent derived unit was added in 1999. Recent changes are directed toward defining base units in terms of invariant constants of physics to provide more precise realisations of units for advances in science and industry.
The modern metric system consists of four electromechanical base units representing four fundamental dimensions of measure: length, mass, time and electromagnetism. The units are:
Together they are sufficient for measuring any known quantity, without reference to further quantities or phenomena.
Three supplemental base units have been defined, but these are not independent since they can be specified entirely in terms of the above four base units: the kelvin, a thermodynamic measure; the candela, a measure of irradiance; and the mole, representing a quantity of substance.
There are currently 22 derived units with special names in the metric system, these are defined in terms of the base units or other named derived units.
Eight of these units are electromagnetic quantities:
Four of these units are mechanical quantities:
Five units represent measures of electromagnetic radiation:
Two units are measures of circular arcs and spherical surfaces:
Three units are miscellaneous:
Although SI, as published by the CGPM, should, in theory, meet all the requirements of commerce, science, and technology, certain customary units of measure have acquired established positions within the world community. In order that such units are used consistently around the world, the CGPM catalogued such units in Tables 6 to 9 of the SI brochure. These categories are:
The SI symbols for the metric units are intended to be identical, regardless of the language used but unit names are
Variations are also found with the spelling of unit names in countries using the same language, including differences in
In SI, which is a coherent system, the unit of power is the "
The concept of coherence was only introduced into the metric system in the third quarter of the 19th century; in its original form the metric system was non-coherent—in particular the