The General Conference on Weights and Measures, abbreviated CGPM from its French name, Conférence générale des poids et mesures, is the plenary, policy-making body and supreme authority of the International Bureau of Weights and Measures (BIPM). Composed of representatives of its Member States, it makes scientific, institutional and financial decisions concerning international metrology. Its best-known responsibility is establishing and revising the International System of Units (SI), the internationally agreed framework for expressing measurements. The conference normally meets every four to six years. (bipm.org)
Origins and legal foundation
The CGPM derives its authority from the Metre Convention, signed in Paris on 20 May 1875 by representatives of seventeen nations. The treaty established a permanent international organization to coordinate measurement standards and provided a three-part institutional structure: the General Conference, the International Committee for Weights and Measures (CIPM), and the BIPM. Article 3 places the bureau under the committee’s direction and supervision, while making the committee answerable to the conference. (bipm.org)
The first CGPM met in 1889 and sanctioned international prototypes for the metre and kilogram. These physical standards supplied common references for length and mass, enabling national standards to be compared against internationally accepted artifacts. The system subsequently expanded beyond its initial emphasis on these two quantities. Revision of the convention in 1921 extended the BIPM’s responsibilities into additional fields of physics, helping create the institutional foundation for a broader system of measurement units. (nist.gov)
Organization and decision-making
The conference represents governments rather than serving as a conventional scientific congress. Under the regulations annexed to the Metre Convention, each participating Member State has one vote. The CIPM convenes the conference, which receives its report on work accomplished and considers measures needed to improve and disseminate the metric system. The regulations require a meeting at least once every six years. (bipm.org)
The CGPM elects the CIPM’s eighteen members, each of a different nationality. Unlike government delegates to the conference, committee members undertake their responsibilities as individuals within the framework established by the CGPM, rather than acting on behalf of their governments. The CIPM supervises the bureau, coordinates shared metrological work and prepares proposals for conference decisions. Its consultative committees provide specialist input on particular measurement fields. (bipm.org)
Conference decisions extend beyond unit definitions. They address the BIPM’s financial provision, institutional objectives and participation arrangements. For example, the 27th meeting adopted resolutions concerning digital transformation, universal adherence to the convention and the bureau’s financial provision for 2024–2027. These responsibilities distinguish the CGPM’s policy-making role from the continuing technical and administrative work of the committee and bureau. (bipm.org)
Establishment and evolution of the SI
The conference’s decisions progressively transformed an artifact-based metric framework into the modern SI. In 1948 it initiated studies toward an international system; in 1954 it selected six base units. The 11th CGPM formally established the SI in 1960, adopting its name and abbreviation alongside rules governing prefixes and other units. In 1971, the mole, measuring amount of substance, became the seventh base unit. (nist.gov)
Successive resolutions replaced several older definitions with reproducible physical references. In 1967, the second was redefined through the caesium-133 hyperfine transition, using 9,192,631,770 periods of the corresponding radiation. In 1983, the metre was defined through the distance travelled by light in vacuum during a specified fraction of a second, fixing the speed of light at exactly 299,792,458 metres per second. (nist.gov)
At its 26th meeting in 2018, the CGPM adopted a revised SI that took effect on 20 May 2019. The revision fixed exact numerical values for the Planck constant, elementary charge, Boltzmann constant and Avogadro constant. These changes supplied new definitions for the kilogram, ampere, kelvin and mole, respectively, within a unified system based on seven defining constants. The kilogram consequently ceased to be defined by the mass of the international prototype. (bipm.org)
Measurement comparability and practical implementation
Internationally agreed definitions provide reference points for calibration and metrological traceability. Traceability connects a measurement result to a reference through a documented, uninterrupted sequence of calibrations, each contributing to its measurement uncertainty. A shared unit definition alone does not establish traceability: the calibration chain and its uncertainties must also be documented. (jcgm.bipm.org)
The wider measurement framework supports confidence in the equivalence of results across laboratories and countries. It combines international references with national metrology institutes, laboratory calibration and arrangements recognizing measurement capabilities. Such comparability can reduce technical barriers to trade arising from incompatible measurement results. The CGPM’s unit and policy decisions form part of this framework rather than replacing its practical laboratory procedures. (bipm.org)
Recent resolutions and scheduled meetings
The 27th CGPM met on 15–18 November 2022. It extended SI prefixes by introducing ronna (R, 10²⁷), quetta (Q, 10³⁰), ronto (r, 10⁻²⁷) and quecto (q, 10⁻³⁰). The resolution cited growing requirements for expressing very large and very small quantities, including digital information. (bipm.org)
Another 2022 resolution addressed Coordinated Universal Time (UTC). It decided that the permitted difference between UTC and Earth-rotation time, UT1, would increase in or before 2035, and requested proposals for implementation and long-term continuity. This was a decision about future timekeeping arrangements, not an immediate alteration of UTC. (bipm.org)
As of 4 October 2026, the 28th CGPM is scheduled for 13–15 October 2026 in Versailles, France. Preparatory materials include draft resolutions and proposed rules of procedure; these remain proposals pending the conference’s consideration. (bipm.org)