Metric system

Four metric measuring devices: a tape measure in centimetres, a thermometer in degrees Celsius, a kilogram weight and a multimeter that measures volts, amperes and ohms

The metric system is an internationally adopted decimal system of measurement. It is in widespread use, and where it is used, it is the only or most common system of weights and measures. It is now known as the International System of Units (SI). It is used to measure everyday things such as the mass of a sack of flour, the height of a person, the speed of a car, and the volume of fuel in its tank. It is also used in science, industry and trade.

In its modern form, it consists of a set of electromechanical base units including metre for length, kilogram for mass, second for time and ampere as an electrical unit, and a few others, which together with their derived units, can measure any useful quantity. Metric system may also refer to other systems of related base and derived units defined before the middle of the 20th century, some of which are still in limited use today.

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 coherent system, which means that its units do not introduce conversion factors not already present in equations relating quantities. It has a property called rationalisation that eliminates certain constants of proportionality in equations of physics.

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 British imperial system and the US customary system of weights and measures, in those systems some or all of the units are now defined in terms of the metric system, such as the US foot which is now a defined decimal fraction of a metre.

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.


Base units

The modern metric system consists of four electromechanical base units representing four fundamental dimensions of measure: length, mass, time and electromagnetism. The units are:

  • the metre for length
  • kilogram for mass
  • second for time
  • ampere for electromagnetism

Together they are sufficient for measuring any known quantity,[1] 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.

Derived units with special names

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:

  • volt, a unit of electrical potential
  • ohm, a unit of electrical resistance
  • tesla, a unit of magnetic flux density
  • weber, a unit of magnetic flux
  • farad, a unit of electrical capacitance
  • henry, a unit of electrical inductance
  • siemens, a unit of electrical conductance (the inverse of ohm)
  • coulomb, a unit of electrical charge

Four of these units are mechanical quantities:

  • watt, a unit of mechanical or electrical power
  • newton, a unit of mechanical force
  • joule, a unit of mechanical, electrical or thermodynamic energy
  • pascal, a unit of pressure

Five units represent measures of electromagnetic radiation:

  • becquerel, a unit of radioactive decay
  • sievert, a unit of absorbed ionising radiation
  • gray, a unit of ionising radiation
  • lux, a unit of luminous flux
  • lumen, a unit of luminous intensity

Two units are measures of circular arcs and spherical surfaces:

  • radian, a unit of circular arc
  • steradian, a unit of spherical surface area

Three units are miscellaneous:

  • degree Celsius, a unit of thermodynamic temperature
  • katal, a unit of catalytic activity (enzymatic)
  • hertz, a unit of cycles per second (inverse of second)

Auxiliary and accessory units

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:[2]

  • Non-SI units accepted for use with the International System of Units (Table 6). This list includes the hour and minute, the angular measures (degree, minute and second of arc), and the historic [non-coherent] metric units, the litre, tonne and hectare (originally agreed by the CGPM in 1879)
  • Non-SI units whose values in SI units must be obtained experimentally (Table 7). This list includes various units of measure used in atomic and nuclear physics and in astronomy such as the dalton, the electron mass, the electron volt, the astronomical unit, the solar mass, and a number of other units of measure that are well-established, but dependent on experimentally-determined physical quantities.
  • Other non-SI units (Table 8). This list catalogues a number of units of measure that have been used internationally in certain well-defined spheres including the bar for pressure, the ångström for atomic physics, the nautical mile and the knot in navigation.
  • Non-SI units associated with the CGS and the CGS-Gaussian system of units (Table 9). This table catalogues a number of units of measure based on the CGS system and dating from the nineteenth century. They appear frequently in the literature, but their continued use is discouraged by the CGPM.

The SI symbols for the metric units are intended to be identical, regardless of the language used[3] but unit names are ordinary nouns and use the character set and follow the grammatical rules of the language concerned. For example, the SI unit symbol for kilometre is "km" everywhere in the world, even though the local language word for the unit name may vary. Language variants for the kilometre unit name include: chilometro (Italian), Kilometer (German),[Note 1] kilometer (Dutch), kilomètre (French), χιλιόμετρο (Greek), quilómetro/quilômetro (Portuguese), kilómetro (Spanish) and километр (Russian).[4][5]

Variations are also found with the spelling of unit names in countries using the same language, including differences in American English and British spelling. For example, meter and liter are used in the United States whereas metre and litre are used in other English-speaking countries. In addition, the official US spelling for the rarely used SI prefix for ten is deka. In American English the term metric ton is the normal usage whereas in other varieties of English tonne is common. Gram is also sometimes spelled gramme in English-speaking countries other than the United States, though this older usage is declining.[6]

In SI, which is a coherent system, the unit of power is the "watt", which is defined as "one joule per second".[7] In the US customary system of measurement, which is non-coherent, the unit of power is the "horsepower", which is defined as "550 foot-pounds per second" (the pound in this context being the pound-force).[8] Similarly, neither the US gallon nor the imperial gallon is one cubic foot or one cubic yard— the US gallon is 231 cubic inches and the imperial gallon is 277.42 cubic inches.[9]

The concept of coherence was only introduced into the metric system in the third quarter of the 19th century;[10] in its original form the metric system was non-coherent—in particular the litre was 0.001 m3 and the are (from which the hectare derives) was 100 m2. However the units of mass and length were related to each other through the physical properties of water, the gram having been designed as being the mass of one cubic centimetre of water at its freezing point.[11]

Other Languages
العربية: نظام متري
azərbaycanca: Metr sistemi
беларуская (тарашкевіца)‎: Мэтрычная сыстэма мер
Cymraeg: System fetrig
贛語: 公制
한국어: 미터법
Bahasa Indonesia: Sistem metrik
íslenska: Metrakerfið
Bahasa Melayu: Sistem metrik
မြန်မာဘာသာ: မက်ထရစ်စနစ်
Nederlands: Metriek stelsel
日本語: メートル法
norsk nynorsk: Metersystemet
Plattdüütsch: Metrisch Eenheitensystem
português: Sistema métrico
română: Sistemul metric
саха тыла: Метр системата
Simple English: Metric system
slovenčina: Metrická sústava
slovenščina: Metrični sistem enot
srpskohrvatski / српскохрватски: Metarski sistem
svenska: Metersystemet
Türkçe: Metrik sistem
українська: Метрична система
Tiếng Việt: Hệ mét
粵語: 公制
中文: 米制