Argentina (1877)
Australia (1947)
Austria (1875)^{ [n1 1] }
Belarus (2020)
Belgium (1875)
Brazil (1921)
Bulgaria (1911)
Canada (1907)
Chile (1908)
China (1977)
Colombia (2012)
Costa Rica (2022)
Croatia (2008)
Czech Republic (1922)^{ [n1 2] }
Denmark (1875)
Ecuador (2019)
Egypt (1962)
Estonia (2021)
Finland (1913)
France (1875)
Germany (1875)^{ [n1 3] }
Greece (2001)
Hungary (1925)^{ [n1 1] }
India (1880)
Indonesia (1960)
Iran (1975)
Iraq (2013)
Ireland (1925)
Israel (1985)
Italy (1875)
Japan (1885)
Kazakhstan (2008)
Kenya (2010)
Lithuania (2015)
Malaysia (2001)
Mexico (1890)
Montenegro (2018)
Morocco (2019)
Netherlands (1929)
New Zealand (1991)
Norway (1875)^{ [n1 4] }
Pakistan (1973)
Poland (1925)
Portugal (1876)
Romania (1884)
Russia (1875)^{ [n1 5] }
Saudi Arabia (2011)
Serbia (2001)
Singapore (1994)
Slovakia (1922)^{ [n1 2] }
Slovenia (2016)
South Africa (1964)
South Korea (1959)
Spain (1875)
Sweden (1875)^{ [n1 4] }
Switzerland (1875)
Thailand (1912)
Tunisia (2012)
Turkey (1875)^{ [n1 6] }
Ukraine (2018)
United Arab Emirates (2015)
United Kingdom (1884)
United States (1878)
Uruguay (1908)
Cameroon (1970–2012)
North Korea (1982–2012)
Venezuela (1879–1907, 1960–2018)
At the 21st meeting of the CGPM in October 1999, the category of "associate" was created for states not yet BIPM members and for economic unions.^{ [15] }
Albania (2007)
Azerbaijan (2015)
Bangladesh (2010)
Bolivia (2008)
Bosnia and Herzegovina (2011)
Botswana (2012)
Cambodia (2021)
Caribbean Community (2005)
Chinese Taipei (2002)
Ethiopia (2018)
Georgia (2008)
Ghana (2009)
Hong Kong (2000)
Jamaica (2003)
Kuwait (2018)
Latvia (2001)
Luxembourg (2014)
Malta (2001)
Mauritius (2010)
Moldova (2007)
Mongolia (2013)
Namibia (2012)
North Macedonia (2006)
Oman (2012)
Panama (2003)
Paraguay (2009)
Peru (2009)
Philippines (2002)
Qatar (2016)
Sri Lanka (2007)
Syria (2012)
Tanzania (2018)
Uzbekistan (2018)
Vietnam (2003)
Zambia (2010)
Zimbabwe (2010–2020, 2022)
Cuba (2000–2021)
Seychelles (2010–2021)
Sudan (2014–2021)
1st^{ [16] } (1889)  The international prototype of the kilogram (IPK), a cylinder made of platinum–iridium, and the international prototype of the metre, an Xcrosssection bar also made from platinum–iridium, were selected from batches manufactured by the British firm Johnson Matthey. Working copies of both artifacts were also selected by lot and other copies distributed to member nations, again by lot. The prototypes and working copies were deposited at the International Bureau of Weights and Measures (Bureau international des poids et mesures), SaintCloud, France. 
2nd^{ [17] } (1895)  No resolutions were passed by the 2nd CGPM. 
3rd^{ [18] } (1901)  The litre was redefined as volume of 1 kg of water. Clarified that kilograms are units of mass, "standard weight" defined, standard acceleration of gravity defined endorsing use of grams force and making them welldefined. 
4th^{ [19] } (1907)  The carat was defined as 200 mg. 
5th^{ [20] } (1913)  The International Temperature Scale was proposed. 
6th^{ [21] } (1921)  The Metre Convention revised. 
7th^{ [22] } (1927)  The Consultative Committee for Electricity (CCE) created. 
8th^{ [23] } (1933)  The need for absolute electrical unit identified. 
9th^{ [24] } (1948)  The ampere, bar, coulomb, farad, henry, joule, newton, ohm, volt, watt, weber were defined. The degree Celsius was selected from three names in use as the name of the unit of temperature. The symbol l (lowercase L) was adopted as symbol for litre. Both the comma and dot on a line are accepted as decimal marker symbols. Symbols for the stere and second changed.^{ [25] } The universal return to the Long Scale numbering system was proposed but not adopted. 
10th^{ [26] } (1954)  The kelvin, standard atmosphere defined. Work on the International System of Units (metre, kilogram, second, ampere, kelvin, candela) began. 
11th^{ [27] } (1960)  The metre was redefined in terms of wavelengths of light. The Units hertz, lumen, lux, tesla were adopted. The new MKSAbased metric system given the official symbol SI for Système International d'Unités and launched as the "modernized metric system". The prefixes pico , nano , micro , mega , giga and tera were confirmed. 
12th^{ [28] } (1964)  The original definition of litre = 1 dm^{3} restored. The prefixes atto and femto were adopted. 
13th^{ [29] } (1967)  The second was redefined as duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium133 atom at a temperature of 0 K. The Degree Kelvin renamed kelvin and the candela redefined. 
14th^{ [30] } (1971)  A new SI base unit, the mole defined. The names pascal and siemens as units of pressure and electrical conductance were approved. 
15th^{ [31] } (1975)  The prefixes peta and exa were adopted. The units gray and becquerel were adopted as radiological units within SI. 
16th^{ [32] } (1979)  The candela and sievert were defined. Both l and L provisionally allowed as symbols for litre. 
17th^{ [33] } (1983)  The metre was redefined in terms of the speed of light, i.e The metre is the length of the path travelled by light in vacuum during a time interval of 1/299,792,458 of a second. 
18th^{ [34] } (1987)  Conventional values were adopted for Josephson constant, K_{J}, and von Klitzing constant, R_{K}, preparing the way for alternative definitions of the ampere and kilogram. 
19th^{ [35] } (1991)  New prefixes yocto , zepto , zetta and yotta were adopted. 
20th^{ [36] } (1995)  The SI supplementary units (radian and steradian) become derived units. 
21st^{ [37] } (1999)  A new SI derived unit, the katal = mole per second, was adopted as the SI unit of catalytic activity. 
22nd^{ [38] } (2003)  A comma or a dot on a line are reaffirmed as decimal marker symbols, and not as grouping symbols in order to facilitate reading; "numbers may be divided in groups of three in order to facilitate reading; neither dots nor commas are ever inserted in the spaces between groups".^{ [39] } 
23rd^{ [40] } (2007)  The definition of the kelvin was clarified and thoughts about possible revision of certain base units discussed. 
24th^{ [41] } (2011)  Proposal to revise the definitions of the SI units, including redefining the kilogram in relation to the Planck constant were accepted in principle, subject to certain technical criteria having been met. 
25th^{ [42] } (2014)  Redefining the kilogram in relation to the Planck constant was discussed but not decided on. Progress towards realising the redefinition has been noted. However, it was concluded that the data did not yet appear to be sufficiently robust. Continued effort on improving the data has been encouraged, such that a resolution that would replace the current definition with the revised definition can be adopted at the 26th meeting. 
26th^{ [43] } (2018)  The kilogram, ampere, kelvin, and mole were redefined^{ [44] } at this meeting, in terms new permanently fixed values of the Planck constant, elementary charge, Boltzmann constant and Avogadro constant, respectively. 
27th^{ [45] } (2022)  New prefixes quecto , ronto , ronna , and quetta were adopted. Planning was begun to eliminate the leap second and stabilize DUT1 by 2035. 
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The International Committee for Weights and Measures consists of eighteen persons, each of a different nationality.^{ [46] } elected by the General Conference on Weights and Measures (CGPM) whose principal task is to promote worldwide uniformity in units of measurement by taking direct action or by submitting proposals to the CGPM.
The CIPM meets every year (since 2011 in two sessions per year) at the Pavillon de Breteuil where, among other matters, it discusses reports presented to it by its Consultative Committees. Reports of the meetings of the CGPM, the CIPM, and all the Consultative Committees, are published by the BIPM.
The secretariat is based in SaintCloud, HautsdeSeine, France.
In 1999 the CIPM has established the CIPM Arrangement de reconnaissance mutuelle (Mutual Recognition Arrangement, MRA) which serves as the framework for the mutual acceptance of national measurement standards and for recognition of the validity of calibration and measurement certificates issued by national metrology institutes.
A recent focus area of the CIPM has been the revision^{ [47] } of the SI.
The CIPM has set up a number of consultative committees (CC) to assist it in its work. These committees are under the authority of the CIPM. The president of each committee, who is expected to take the chair at CC meetings, is usually a member of the CIPM. Apart from the CCU, membership of a CC is open to National Metrology Institutes (NMIs) of Member States that are recognized internationally as most expert in the field.^{ [48] } NMIs from Member States that are active in the field, but lack the expertise to become Members, are able to attend CC meetings as observers.^{ [49] }
These committees are:^{ [48] }
The CCU's role is to advise on matters related to the development of the SI and the preparation of the SI brochure.^{ [48] } It has liaison with other international bodies such as International Organization for Standardization (ISO), International Astronomical Union (IAU), International Union of Pure and Applied Chemistry (IUPAC), International Union of Pure and Applied Physics (IUPAP) and International Commission on Illumination (CIE).^{ [Note 4] }
Official reports of the CIPM include:^{ [50] }
From time to time the CIPM has been charged by the CGPM to undertake major investigations related to activities affecting the CGPM or the BIPM. Reports produced include:^{ [51] }
The Blevin Report, published in 1998, examined the state of worldwide metrology.^{ [52] } The report originated from a resolution passed at the 20th CGPM (October 1995) which committed the CIPM to
study and report on the longterm national and international needs relating to metrology, the appropriate international collaborations and the unique role of the BIPM to meet these needs, and the financial and other commitments that will be required from the Member States in the coming decades.
The report identified, amongst other things, a need for closer cooperation between the BIPM and other organisations such as International Organization of Legal Metrology (OIML) and International Laboratory Accreditation Cooperation (ILAC) with clearly defined boundaries and interfaces between the organisations. Another major finding was the need for cooperation between accreditation laboratories and the need to involve developing countries in the world of metrology.
The Kaarls Report^{ [53] } published in 2003 examined the role of the BIPM in the evolving needs for metrology in trade, industry and society.
The CIPM has responsibility for commissioning the SI brochure, which is the formal definition of the International system of units. The brochure is produced by the CCU in conjunction with a number of other international organisations. Initially the brochure was only in French – the official language of the metre convention, but recent versions have been published simultaneously in both English and French, with the French text being the official text. The 6th edition was published in 1991,^{ [54] } the 7th edition was published in 1998, and the 8th, in 2006.^{ [55] } The most recent edition is the 9th edition, originally published as version 1 in 2019 to include the 2019 redefinition of the SI base units (aka "new SI"); it was updated to version 2 in December 2022 to also include the new SI prefixes ronna, quetta, ronto and quecto introduced in November 2022.^{ [56] }
The International Bureau of Weights and Measures is an intergovernmental organisation, through which its 59 memberstates act together on measurement standards in four areas: chemistry, ionising radiation, physical metrology, and coordinated universal time. It is based in SaintCloud, Paris, France. The organisation has been referred to as IBWM in older literature.
The kilogram is the unit of mass in the International System of Units (SI), having the unit symbol kg. It is a widely used measure in science, engineering and commerce worldwide, and is often simply called a kilo colloquially. It means 'one thousand grams'.
The litre or liter is a metric unit of volume. It is equal to 1 cubic decimetre (dm^{3}), 1000 cubic centimetres (cm^{3}) or 0.001 cubic metre (m^{3}). A cubic decimetre occupies a volume of 10 cm × 10 cm × 10 cm and is thus equal to onethousandth of a cubic metre.
The metre or meter, symbol m, is the primary unit of length in the International System of Units (SI), though its prefixed forms are also used relatively frequently.
The Metre Convention, also known as the Treaty of the Metre, is an international treaty that was signed in Paris on 20 May 1875 by representatives of 17 nations. The treaty created the International Bureau of Weights and Measures (BIPM), an intergovernmental organization under the authority of the General Conference on Weights and Measures (CGPM) and the supervision of the International Committee for Weights and Measures (CIPM), that coordinates international metrology and the development of the metric system.
The International System of Units, known by the international abbreviation SI in all languages and sometimes pleonastically as the SI system, is the modern form of the metric system and based on the metre as the unit of length and either the kilogram as the unit of mass or the kilogramforce as the unit of force.</ref> and the world's most widely used system of measurement. Established and maintained by the General Conference on Weights and Measures (CGPM), it is the only system of measurement with an official status in nearly every country in the world, employed in science, technology, industry, and everyday commerce.
The SI base units are the standard units of measurement defined by the International System of Units (SI) for the seven base quantities of what is now known as the International System of Quantities: they are notably a basic set from which all other SI units can be derived. The units and their physical quantities are the second for time, the metre for length or distance, the kilogram for mass, the ampere for electric current, the kelvin for thermodynamic temperature, the mole for amount of substance, and the candela for luminous intensity. The SI base units are a fundamental part of modern metrology, and thus part of the foundation of modern science and technology.
Metrology is the scientific study of measurement. It establishes a common understanding of units, crucial in linking human activities. Modern metrology has its roots in the French Revolution's political motivation to standardise units in France when a length standard taken from a natural source was proposed. This led to the creation of the decimalbased metric system in 1795, establishing a set of standards for other types of measurements. Several other countries adopted the metric system between 1795 and 1875; to ensure conformity between the countries, the Bureau International des Poids et Mesures (BIPM) was established by the Metre Convention. This has evolved into the International System of Units (SI) as a result of a resolution at the 11th General Conference on Weights and Measures (CGPM) in 1960.
The gram is a unit of mass in the International System of Units (SI) equal to one one thousandth of a kilogram.
Vienna Standard Mean Ocean Water (VSMOW) is an isotopic standard for water. Despite the name, VSMOW is pure water with no salt or other chemicals found in the oceans. The VSMOW standard was promulgated by the International Atomic Energy Agency in 1968, and since 1993 continues to be evaluated and studied by the IAEA along with the European Institute for Reference Materials and Measurements and the American National Institute of Standards and Technology. The standard includes both the established values of stable isotopes found in waters and calibration materials provided for standardization and interlaboratory comparisons of instruments used to measure these values in experimental materials.
The Pavillon de Breteuil is the headquarters of the International Bureau of Weights and Measures (BIPM). The building lies in the southeastern section of the Parc de SaintCloud in SaintCloud, France, to the west of Paris. It is listed in France as an historic monument.
France has a unique history of units of measurement due to its radical decision to invent and adopt the metric system after the French Revolution.
The International Commission on Radiation Units and Measurements (ICRU) is a standardization body set up in 1925 by the International Congress of Radiology, originally as the XRay Unit Committee until 1950. Its objective "is to develop concepts, definitions and recommendations for the use of quantities and their units for ionizing radiation and its interaction with matter, in particular with respect to the biological effects induced by radiation".
The kelvin, symbol K, is the primary unit of temperature in the International System of Units (SI), used alongside its prefixed forms and the degree Celsius. It is named after the Belfastborn and University of Glasgowbased engineer and physicist William Thomson, 1st Baron Kelvin (1824–1907). The Kelvin scale is an absolute thermodynamic temperature scale, meaning it uses absolute zero as its null (zero) point.
The history of the metre starts with the Scientific Revolution that is considered to have begun with Nicolaus Copernicus's publication of De revolutionibus orbium coelestium in 1543. Increasingly accurate measurements were required, and scientists looked for measures that were universal and could be based on natural phenomena rather than royal decree or physical prototypes. Rather than the various complex systems of subdivision then in use, they also preferred a decimal system to ease their calculations.
In metrology, a standard is an object, system, or experiment that bears a defined relationship to a unit of measurement of a physical quantity. Standards are the fundamental reference for a system of weights and measures, against which all other measuring devices are compared. Historical standards for length, volume, and mass were defined by many different authorities, which resulted in confusion and inaccuracy of measurements. Modern measurements are defined in relationship to internationally standardized reference objects, which are used under carefully controlled laboratory conditions to define the units of length, mass, electrical potential, and other physical quantities.
In 2019, four of the seven SI base units specified in the International System of Quantities were redefined in terms of natural physical constants, rather than human artifacts such as the standard kilogram. Effective 20 May 2019, the 144th anniversary of the Metre Convention, the kilogram, ampere, kelvin, and mole are now defined by setting exact numerical values, when expressed in SI units, for the Planck constant, the elementary electric charge, the Boltzmann constant, and the Avogadro constant, respectively. The second, metre, and candela had previously been redefined using physical constants. The four new definitions aimed to improve the SI without changing the value of any units, ensuring continuity with existing measurements. In November 2018, the 26th General Conference on Weights and Measures (CGPM) unanimously approved these changes, which the International Committee for Weights and Measures (CIPM) had proposed earlier that year after determining that previously agreed conditions for the change had been met. These conditions were satisfied by a series of experiments that measured the constants to high accuracy relative to the old SI definitions, and were the culmination of decades of research.
The history of the metric system began during the Age of Enlightenment with measures of length and weight derived from nature, along with their decimal multiples and fractions. The system became the standard of France and Europe within half a century. Other measures with unity ratios were added, and the system went on to be adopted across the world.
The following outline is provided as an overview of and topical guide to the metric system – various loosely related systems of measurement that trace their origin to the decimal system of measurement introduced in France during the French Revolution.
Carlos Ibáñez e Ibáñez de Ibero, 1st Marquis of Mulhacén, was a Spanish divisional general and geodesist. He represented Spain at the 1875 Conference of the Metre Convention and was the first president of the International Committee for Weights and Measures. As a forerunner geodesist and president of the International Geodetic Association, he played a leading role in the worldwide dissemination of the metric system. His activities resulted in the distribution of a platinum and iridium prototype of the metre to all States parties to the Metre Convention during the first meeting of the General Conference on Weights and Measures in 1889. These prototypes defined the metre right up until 1960.
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