International Prototype of the Kilogram

A platinum–iridium cylinder kept near Paris that served as the kilogram’s international reference from 1889 until the unit was redefined in 2019 using the Planck constant.

General Conference on Weights and Measures: The international body that adopts resolutions on the International System of Units and related metrology matters. It approved the prototype as the kilogram’s international definition in 1889.
Kilogram: The SI base unit of mass, defined since 2019 by fixing the numerical value of the Planck constant. The prototype embodied the kilogram’s former artifact-based definition.
Kilogram No. 20: A national prototype kilogram kept by the United States as its primary mass standard before the 2019 redefinition. Its comparisons with the international prototype show how the U.S. standard was maintained.
Natural kilogram: A proposed mass unit based on a natural physical quantity rather than an individual artifact. Proposals for a natural standard addressed the prototype’s dependence on one object.
International Bureau of Weights and Measures: An intergovernmental organization that maintains international measurement standards and coordinates metrology. It keeps the prototype and coordinates comparisons with national standards.
2019 redefinition of the SI base units: The revision that defined several SI units through fixed values of fundamental constants, effective 20 May 2019. It replaced the prototype’s role with a definition anchored to the Planck constant.
Kilogram No. 39: A national prototype kilogram associated with the United Kingdom’s mass standard system. It exemplifies a national copy periodically compared against the international artifact.
Gravimetric definition of the kilogram: A proposed definition of mass through the weight of a specified volume of water. It was an earlier alternative to defining the kilogram by a metal artifact.
National prototype of the kilogram: A national mass standard made to reproduce the kilogram and compared periodically with the international prototype. These copies carried the prototype’s assigned mass into national measurement systems.
Planck constant: A fundamental constant relating the energy of a photon to its frequency. Its fixed numerical value now anchors the kilogram’s definition.

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