## Metric system |

The **metric system** is an internationally adopted

In its modern form, it consists of a set of *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 *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 *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.

- units
- realisation of units
- properties as a system
- international system of units
- conversion and calculation incidents
- conversion table
- see also
- notes
- references
- external links

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.

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)

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, thelitre ,tonne andhectare (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 thedalton , theelectron mass , theelectron volt , theastronomical unit , thesolar 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 thebar for pressure, theångström foratomic physics , thenautical mile and theknot innavigation .**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 *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 *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 *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 "*one* joule per second".^{[7]} In the ^{[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 ^{3} and the ^{2}. 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

asturianu: Sistema Métricu Decimal

azərbaycanca: Metr sistemi

беларуская: Метрычная сістэма мер

беларуская (тарашкевіца): Мэтрычная сыстэма мер

bosanski: Metrički sistem

català: Sistema mètric decimal

čeština: Metrická soustava

Cymraeg: System fetrig

Deutsch: Metrisches Einheitensystem

eesti: Meetermõõdustik

español: Sistema métrico decimal

Esperanto: Metra decimala sistemo

euskara: Sistema metriko hamartarra

فارسی: واحدهای اندازهگیری متریک

Frysk: Metryk stelsel

Gaeilge: Córas méadrach

galego: Sistema métrico

贛語: 公制

한국어: 미터법

हिन्दी: मीटरी पद्धति

Bahasa Indonesia: Sistem metrik

íslenska: Metrakerfið

עברית: השיטה המטרית

қазақша: Бірліктер жүйесі

Latina: Mensura metrica

മലയാളം: മെട്രിക് അളവുകൾ

मराठी: मेट्रिक पद्धती

Bahasa Melayu: Sistem metrik

မြန်မာဘာသာ: မက်ထရစ်စနစ်

Nederlands: Metriek stelsel

日本語: メートル法

norsk nynorsk: Metersystemet

Patois: Mechrik Sistim

Plattdüütsch: Metrisch Eenheitensystem

português: Sistema métrico

română: Sistemul metric

русский: Метрическая система мер

саха тыла: Метр системата

Scots: Metric seestem

Simple English: Metric system

سنڌي: ميٽرڪ نظام

slovenčina: Metrická sústava

slovenščina: Metrični sistem enot

srpskohrvatski / српскохрватски: Metarski sistem

suomi: Metrijärjestelmä

svenska: Metersystemet

Tagalog: Sistemang metriko

தமிழ்: மெட்ரிக் முறை

తెలుగు: మెట్రిక్ పద్ధతి

Türkçe: Metrik sistem

українська: Метрична система

اردو: میٹرک نظام

Tiếng Việt: Hệ mét

Winaray: Sistema Metriko

ייִדיש: מעטרישע סיסטעם

Yorùbá: Sístẹ̀mù ìwọ̀n mítà

粵語: 公制

中文: 米制