The Metre Convention: 150 Years of Global Measurement Cooperation
Source: BIPM. Published: 2025-07-01.
The Chaos Before 1875
Before the Metre Convention was signed, the world of measurement was a patchwork of local and national standards that made international trade, science, and engineering unnecessarily difficult. France had the metric system, adopted during the Revolution. Britain used imperial units. The German states each had their own traditional measures, as did the Italian states and the countries of central and eastern Europe. A foot in one country was not the same length as a foot in another. Platinum metre bars kept in national archives agreed with each other only approximately, and that approximation deteriorated as measurement technology improved and the discrepancies became visible. The scientific and commercial communities increasingly recognised that a unified, internationally agreed system of measurement was not merely convenient — it was a prerequisite for modern civilisation.
The Metre Convention of 1875
On 20 May 1875, representatives of seventeen nations gathered in Paris to sign the Metre Convention — also known as the Treaty of the Metre — establishing the International Bureau of Weights and Measures (BIPM) and the international framework for measurement. The signatories agreed to maintain the new system of metric units, to share scientific and technical developments, and to fund a permanent international organisation headquartered at Sèvres, near Paris, that would serve as the custodian of the world's measurement standards. The date, May 20, is now celebrated annually as World Metrology Day. The original signatories included France, Germany, the United States, and most major European powers; the treaty has since grown to encompass 64 member states representing the vast majority of global economic activity.
What the BIPM Does
The International Bureau of Weights and Measures serves as the world's metrological authority. It maintains and develops the International System of Units, coordinates comparisons of national measurement standards among member states, publishes the official definition of Coordinated Universal Time (UTC), and organises the international comparisons that ensure measurement results from different countries are mutually consistent. The BIPM is governed by the General Conference on Weights and Measures (CGPM), which meets every four years to adopt formal decisions on SI definitions and related matters, and by the International Committee for Weights and Measures (CIPM), which oversees day-to-day governance. Without the BIPM, a metre measured in Tokyo would not reliably equal a metre measured in Berlin — and the precision manufacturing, international trade, and scientific collaboration that depend on that equality could not function.
The 1960 SI: A Coherent System
The original Metre Convention established the metric system as an international standard, but the formal International System of Units — the SI — was not adopted until 1960, at the 11th General Conference. The 1960 SI formalised seven base units, defined a hierarchy of derived units, and established the naming conventions and SI prefixes that remain in use today. The system was explicitly designed to be coherent: derived units are formed from base units by multiplication and division without numerical conversion factors, eliminating the awkward constants that plague non-SI unit systems. A joule, for example, is defined as exactly one kilogram times one square metre per square second — the SI units combine cleanly without any numerical adjustments.
The 2019 Revision: Constants Over Artefacts
The 2019 revision of the SI, agreed at the 26th General Conference, represented the most fundamental change to the measurement system since 1960. All seven base units are now defined by fixing the numerical values of seven fundamental physical constants: the speed of light, Planck's constant, the elementary charge, the Boltzmann constant, Avogadro's constant, the luminous efficacy of 540 THz radiation, and the hyperfine transition frequency of cesium-133. Physical artefacts — such as the international prototype kilogram — were retired as definitions. The revised SI is artefact-free, invariant, and universally accessible: any laboratory that can realise the relevant constants can independently verify the SI base units without reference to a physical object kept in a vault in France.
Global Reach and the Ongoing Work
Today the Metre Convention has 64 member states and 36 associate states and economies, covering virtually all of global trade and scientific output. The BIPM's ongoing work includes developing the next generation of optical atomic clocks for a future redefinition of the second, improving the realisation of thermodynamic temperature measurement, and coordinating the international comparisons that underpin the traceability chains used by every calibration laboratory in the world. The goal remains what it was in 1875: a single, universal, scientifically rigorous system of measurement that eliminates ambiguity in every technical communication across national, linguistic, and disciplinary boundaries.
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