# Metric system

metricmetric unitsmetric unitmetric measurementscmInternational Metric System International Metric systemdecimal metric systemFrench metric convocationkilo
The metric system is an internationally recognised decimalised system of measurement.wikipedia
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### Metrication

It is in widespread use, and where it is adopted, it is the only or most common system of weights and measures (see metrication).
Metrication or metrification is the act or process of converting the system of measurement traditionally used in a country to the metric system.

### System of measurement

Systems of measurementsystem of unitsHistorical weights and measures
The metric system is an internationally recognised decimalised system of measurement.
Systems of measurement in use include the International System of Units (SI), the modern form of the metric system, the imperial system, and United States customary units.

### International System of Units

SISI unitsSI unit
It is now known as the International System of Units (SI).

### Decimalisation

decimal currencydecimaliseddecimalization
The metric system is an internationally recognised decimalised system of measurement.
Units of physical measurement, such as length and mass, were decimalised with the introduction of the metric system, which has been adopted by almost all countries with the prominent exception of the United States and, to a lesser extent, the United Kingdom.

### Imperial units

imperialimperial systemimperial unit
Units of the British imperial system and the related US customary system are officially defined in terms of the metric system.
By the late 20th century, most nations of the former empire had officially adopted the metric system as their main system of measurement and imperial units are still used in the United Kingdom, Canada and other countries formerly part of the British Empire.

### Dimensional analysis

dimensiondimensionsdimensionally equivalent
As a result, in dimensional analysis, they remain wholly separate concepts.
The conversion of units from one dimensional unit to another is often easier within the metric or SI system than in others, due to the regular 10-base in all units.

### Litre

Lmlliter
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 litre was 0.001 m 3 and the are (from which the hectare derives) was 100 m 2.
The litre (international spelling) or liter (American spelling) (SI symbols L and l, other symbol used: ℓ) is a metric system unit of volume which is a non-SI unit mentioned in the SI.

### Hour

htime of dayhr
Prefixes are not usually used to indicate multiples of a second greater than 1; the non-SI units of minute, hour and day are used instead.
An hour (symbol: h; also abbreviated hr.) is a unit of time conventionally reckoned as 1⁄24 of a day and scientifically reckoned as 3,599–3,601 seconds, depending on conditions.

### Kilometre

kmkilometerkilometers
Language variants for the kilometre unit name include: chilometro (Italian), Kilometer (German), kilometer (Dutch), kilomètre (French), χιλιόμετρο (Greek), quilómetro/quilômetro (Portuguese), kilómetro (Spanish) and километр (Russian). Thus the kilogram and kilometre are a thousand grams and metres respectively, and a milligram and millimetre are one thousandth of a gram and metre respectively.
The kilometre (SI symbol: km; or ), spelt kilometer in American English, is a unit of length in the metric system, equal to one thousand metres (kilo- being the SI prefix for 1000).

### United States customary units

USUS customary unitsU.S. customary units
Units of the British imperial system and the related US customary system are officially defined in terms of the metric system.
The International System of Units (SI), the modern form of the metric system, is preferred for many uses by the U.S. National Institute of Standards and Technology (NIST).

### Metre

metermmetres
In its modern form, it consists of a set of base units: metre for length, kilogram for mass, second for time, ampere for electrical current, kelvin for temperature, candela for luminous intensity and mole for quantity. Notably, as per the International Yard and Pound Agreement the base units of the Imperial and Customary system are defined in terms of the metre and kilogram. Thus the kilogram and kilometre are a thousand grams and metres respectively, and a milligram and millimetre are one thousandth of a gram and metre respectively.
A member of the Preparatory Committee since 1870 and Spanish representative at the Paris Conference in 1875, Carlos Ibáñez e Ibáñez de Ibero intervened with the French Academy of Sciences to rally France to the project to create an International Bureau of Weights and Measures equipped with the scientific means necessary to redefine the units of the metric system according to the progress of sciences.

### Tonne

ttonnesmetric ton
The metre–tonne–second system of units (MTS) was based on the metre, tonne and second – the unit of force was the sthène and the unit of pressure was the pièze.
The tonne ( or ; non-SI unit, symbol: t), commonly referred to as the metric ton in Canada, the United Kingdom and the United States, is a non-SI metric unit of mass equal to 1,000 kilograms or one megagram (symbol: Mg).

### International yard and pound

International Yard and Pound Agreementinternational yardinternational agreement
Notably, as per the International Yard and Pound Agreement the base units of the Imperial and Customary system are defined in terms of the metre and kilogram.
In 1866, the U.S. Congress passed a law that allowed, but did not require, the use of the metric system in trade and commerce.

### Deca-

In addition, the official US spelling for the rarely used SI prefix for ten is deka.
Deca- (International spelling as used by the International Bureau of Weights and Measures ; symbol: da) or deka- (American spelling ) is a decimal unit prefix in the metric system denoting a factor of ten.

### Unit of measurement

unitunits of measurementweights and measures
Designed for transnational use, it consisted of a basic set of units of measurement, now known as base units.
Now there is a global standard, the International System of Units (SI), the modern form of the metric system.

### Kilogram

kgmgmilligram
In its modern form, it consists of a set of base units: metre for length, kilogram for mass, second for time, ampere for electrical current, kelvin for temperature, candela for luminous intensity and mole for quantity. Notably, as per the International Yard and Pound Agreement the base units of the Imperial and Customary system are defined in terms of the metre and kilogram. Thus the kilogram and kilometre are a thousand grams and metres respectively, and a milligram and millimetre are one thousandth of a gram and metre respectively. The kilogram was defined by the mass of a man-made artefact of platinum-iridium held in a laboratory in France, until the new definition was introduced in May 2019.
The kilogram (also kilogramme) is the base unit of mass in the metric system, formally the International System of Units (SI), having the unit symbol kg.

### Gram

ggramsgr
Thus the kilogram and kilometre are a thousand grams and metres respectively, and a milligram and millimetre are one thousandth of a gram and metre respectively.
The gram (alternative spelling: gramme; SI unit symbol: g; gramma, from Greek ) is a metric system unit of mass.

### Hectare

hahectaresare
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 litre was 0.001 m 3 and the are (from which the hectare derives) was 100 m 2.
Some countries that underwent a general conversion from traditional measurements to metric measurements (e.g. Canada) required a resurvey when units of measure in legal descriptions relating to land were converted to metric units.

### Metric prefix

SI prefixunit prefixprefix
Derived units were built up from the base units using logical rather than empirical relationships while multiples and submultiples of both base and derived units were decimal-based and identified by a standard set of prefixes.
Decimal multiplicative prefixes have been a feature of all forms of the metric system, with six of these dating back to the system's introduction in the 1790s.

### 2019 redefinition of the SI base units

2019 redefinition of SI base unitsredefinitionredefinition of the SI base units
The kilogram was defined by the mass of a man-made artefact of platinum-iridium held in a laboratory in France, until the new definition was introduced in May 2019.
The metric system was originally conceived as a system of measurement that was derivable from unchanging phenomena, but practical limitations necessitated the use of artefacts – the prototype metre and prototype kilogram – when the metric system was introduced in France in 1799.

### International Prototype of the Kilogram

International Prototype KilogramIPKkilogram
The replicas were subject to periodic validation by comparison to the original, called the IPK.
The Metre Convention was signed on May 20, 1875 and further formalised the metric system (a predecessor to the SI), quickly leading to the production of the IPK.

### Millimetre

mmmillimetermillimeters
Thus the kilogram and kilometre are a thousand grams and metres respectively, and a milligram and millimetre are one thousandth of a gram and metre respectively.
The millimetre (international spelling as used by the International Bureau of Weights and Measures; SI unit symbol mm) or millimeter (American spelling) is a unit of length in the metric system, equal to one thousandth of a metre, which is the SI base unit of length.

### Centimetre–gram–second system of units

CGScgs unitsCGS unit
The CGS system had two units of energy, the erg that was related to mechanics and the calorie that was related to thermal energy; so only one of them (the erg) could bear a coherent relationship to the base units.
The centimetre–gram–second system of units (abbreviated CGS or cgs) is a variant of the metric system based on the centimetre as the unit of length, the gram as the unit of mass, and the second as the unit of time.

### Unit prefix

myria--xennor xennameter
During the 19th century the prefix myria-, derived from the Greek word μύριοι (mýrioi), was used as a multiplier for 10000.
The prefixes of the metric system precede a basic unit of measure to indicate a decadic multiple and fraction of a unit.

### Calorie

calorieskcalkilocalorie
The CGS system had two units of energy, the erg that was related to mechanics and the calorie that was related to thermal energy; so only one of them (the erg) could bear a coherent relationship to the base units.
Calorie relates directly to the metric system, and therefore to the SI system.