Other meanings of SI Brochure
Metrology
The SI Brochure is the official publication of the International System of Units (SI), issued by the International Bureau of Weights and Measures (BIPM). It defines and explains the SI, including its base units, derived units, prefixes, and the rules for their use. First published in 1970, the brochure is periodically revised to reflect decisions of the General Conference on Weights and Measures (CGPM), most recently in its 9th edition (2019), which implemented the redefinition of the kilogram, ampere, kelvin, and mole in terms of fundamental constants.
The SI Brochure serves as the definitive reference for the International System of Units, providing the official definitions, symbols, and usage conventions that ensure global consistency in measurement. It is intended for scientists, engineers, educators, and legal metrology authorities, and it forms the basis for national standards and international trade. The brochure is published by the BIPM under the authority of the CGPM, and its content is aligned with the decisions of the CGPM and the Consultative Committee for Units (CCU).1
The first edition appeared in 1970, consolidating the SI as established by the 11th CGPM in 1960. Subsequent editions have incorporated major changes, such as the addition of the mole as a base unit in 1971 and the redefinition of the metre in 1983. The 9th edition, published in 2019, is notable for the first time in history that all base units are defined by fundamental constants of nature, eliminating the last physical artifact, the international prototype of the kilogram.2
The brochure is organized into chapters covering the SI base units, derived units with special names, prefixes, and the rules for writing and using SI symbols. It includes a detailed explanation of the defining constants, such as the speed of light and the Planck constant, and provides guidance on expressing quantities in terms of SI units. The 9th edition also introduces a new way of presenting the SI, using a 'symbolic' representation of the defining constants, and includes a section on the future evolution of the SI.3
Beyond the core definitions, the SI Brochure contains several lesser-known details. For instance, it specifies the exact typography for unit symbols, such as the use of roman (upright) type and the rule that symbols are not followed by a period unless at the end of a sentence. It also addresses the use of prefixes with units, including the rarely used prefixes like 'yocto' and 'zetta', and clarifies that the kilogram is the only base unit with a prefix in its name. Additionally, the brochure includes a historical note on the 'international prototype' and the transition to constant-based definitions, and it provides guidance on the use of units outside the SI, such as the litre and the tonne, which are accepted for use with the SI.
The SI Brochure is available in both French and English, with the French version being the official text.
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