International Bureau of Weights and Measures

The International Bureau of Weights and Measures (BIPM) is an intergovernmental organization that coordinates the global measurement system and maintains international standards for units of measurement.

International System of Units: The internationally agreed system of measurement units, built around seven base units and their derived units. The BIPM coordinates the system’s maintenance and publication.
Treaty of the Metre: The 1875 treaty establishing international cooperation on units and creating the BIPM. Its signatories founded the bureau and defined its intergovernmental mandate.
National metrology institute: A national organization that maintains measurement standards and provides traceability and calibration services. These institutes connect the BIPM’s international framework to measurement users in each country.
2019 redefinition of the SI: The 2019 revision of the International System of Units, which defined units through fixed values of physical constants. The BIPM coordinated the transition from artifact-based definitions to constant-based units.
General Conference on Weights and Measures: The governing conference where member states make decisions about the international measurement system. It approves major changes to units and measurement standards coordinated by the BIPM.
International Prototype Metre: A platinum-iridium bar used as the international reference for the metre from 1889 to 1960. It was an early physical standard entrusted to the BIPM’s custody.
Metrological traceability: The property of a measurement result that links it to a reference through a documented, unbroken calibration chain. BIPM comparisons help establish international links among national measurement references.
Planck constant: A fundamental physical constant that relates a photon’s energy to its frequency. Its fixed numerical value now defines the kilogram in the SI.
International Committee for Weights and Measures: The committee that oversees international metrology and supervises the BIPM’s work between general conferences. It guides the BIPM and prepares technical decisions for member states.
International Prototype Kilogram: A platinum-iridium cylinder that served as the international reference for the kilogram from 1889 to 2019. The BIPM safeguarded this prototype while national copies were compared against it.

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