SI units

SI units
Name	SI units
Caption	International System of Units (SI) emblem
Established	1960
Superseded	Metric System (partially)
Governing body	International Bureau of Weights and Measures
Abbreviation	SI

SI units The International System of Units (SI) is the globally accepted system for measurement used in science, engineering, industry, and commerce. It provides a coherent framework linking base quantities to derived quantities through a set of base units, prefixes, and definitions maintained by international organizations. Major institutions such as the International Bureau of Weights and Measures, the General Conference on Weights and Measures, and national metrology institutes coordinate revisions, dissemination, and legal implementation.

History and development

The project to standardize measurements traces to events like the French Revolution reforms and the adoption of the Metre Convention of 1875, which created the International Bureau of Weights and Measures and fostered international cooperation. The metric system evolved through contributions from scientists and policymakers including André-Marie Ampère, Gustave Eiffel (as patron of engineering standards), and committees arising from the General Conference on Weights and Measures; these influenced the 1960 decision to formalize the International System at the 11th CGPM. Subsequent milestones include decisions at later sessions of the General Conference on Weights and Measures and critical work by agencies such as the International Organization for Standardization and the Consultative Committee for Units (CCU). The SI consolidated earlier standards like the metre and kilogram from artifacts and physical prototypes toward definitions anchored in fundamental constants, a transition culminating in the 2019 redefinition ratified by the CGPM.

SI base units

The SI is built on a set of base quantities each associated with a base unit established and maintained by international bodies. Historically, base units drew on artefacts such as the international prototype kilogram preserved at the International Bureau of Weights and Measures; modern practice defines units by fixing numerical values of fundamental constants recognized by the scientific community and endorsed in resolutions of the General Conference on Weights and Measures. National metrology institutes—including the National Institute of Standards and Technology and the Physikalisch-Technische Bundesanstalt—realize these base units in laboratories for dissemination. The coherence of the SI allows base units to combine algebraically into derived units used across disciplines in laboratories and industry.

Derived units and prefixes

Derived units are produced by algebraic combinations of base units to express quantities such as force, pressure, energy, and power; historically named derived units honor figures like Isaac Newton and James Watt through unit names, and many are used alongside SI symbols. For practical use across magnitudes, a system of decimal prefixes ranging from yotta to yocto was formalized by international standards committees and adopted by bodies such as the International Electrotechnical Commission and the International Organization for Standardization. These prefixes facilitate expressing very large or small quantities encountered in contexts from astrophysics associated with institutions like the European Space Agency to nanotechnology research at universities and laboratories such as CERN.

Definitions and standards

Definitions of SI units are established by the General Conference on Weights and Measures following technical input from consultative committees and national laboratories. The modern SI defines units by fixing exact values of fundamental constants—such as the Planck constant and the speed of light—based on recommendations from expert groups including the International Committee for Weights and Measures and the Bureau International des Poids et Mesures technical committees. International standards publications and guidance from organizations like the International Organization for Standardization and the IEEE ensure uniform notation, unit symbols, and practices for calibration, traceability, and measurement uncertainty used by metrology institutes worldwide.

Usage and legal status

Adoption and legal status of the SI vary by jurisdiction; national laws and regulatory frameworks enacted by parliaments or legislatures reference SI units for commerce, safety, and public administration, with enforcement by national standards agencies and metrology institutes. Entities such as the European Commission have directives mandating SI usage in member states, while other countries have phased adoption strategies informed by consultations with institutions like the World Trade Organization and national ministries. Scientific publications, technical standards, and international treaties commonly require SI units to ensure interoperability across organizations including research universities, multinational corporations, and international laboratories.

Revisions and future developments

Revisions to the SI arise from advances in science, technology, and consensus at meetings of the General Conference on Weights and Measures and working groups involving stakeholders such as the International Bureau of Weights and Measures and national metrology institutes. Notable recent developments include the 2019 redefinition based on fixed constants and ongoing work to refine realizations using quantum standards developed in collaborations with institutions like NIST and PTB. Future developments may address practical realizations of units at extreme scales, integration with digital measurement infrastructures promoted by organizations involved in standards harmonization, and continued coordination through international conferences and committees to accommodate emerging scientific and technological needs.

Category:Metrology