History of the metric system

The history of the metric system traces its creation in revolutionary France, its spread through international agreements, and its evolution into the modern International System of Units.

French Revolution: A period of political upheaval in France from 1789 to 1799 that ended the monarchy and reshaped government. The upheaval created the political setting for France’s campaign to standardize weights and measures.
Metre: The SI base unit of length, defined by fixing the speed of light in vacuum. Its changing definitions mark the metric system’s shift from a terrestrial measure to a constant-based standard.
Convention of the Metre: An 1875 international treaty that established cooperation on metric standards and created permanent institutions. It turned metric coordination into a standing international undertaking.
Imperial units: A system of units established in Britain in 1824, including the yard, pound, and gallon. Imperial measures remained a major alternative as metric standards spread across Europe and beyond.
Metrology: The science of measurement, including the realization, evaluation, and comparison of units and standards. The metric system’s continuing development depends on metrology’s methods and institutions.
Charles Maurice de Talleyrand-Périgord: A French diplomat and statesman who proposed international cooperation on standards of measurement in 1790. His 1790 proposal helped launch the project that produced the metric system.
Kilogram: The SI base unit of mass, defined by fixing the numerical value of the Planck constant. Its history captures the long reliance on a physical artifact and the eventual move to a fixed constant.
International Bureau of Weights and Measures: An international organization that maintains measurement standards and coordinates metrology among member states. The bureau provides the institutional continuity behind successive metric standards.
United States customary units: A system of measures used in the United States, derived largely from English units but with distinct definitions. Their continued use illustrates the limits of worldwide metric adoption.
Redefinition of SI base units: The 2019 change that defined all seven SI base units through fixed values of fundamental constants. This revision completed the move away from definitions tied to particular physical objects.

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Pierre MéchainRelated: The meridian survey Méchain joined supplied measurements for the meter’s original definition.