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Preface to the 8th edition
Version française
SI units
Decimal multiples and submultiples of SI units
Units outside the SI
Writing unit symbols and names, and expressing the values of quantities
Decisions of the CGPM and the CIPM
Practical realization of the definitions of some important units
Units for photochemical and photobiological quantitites
List of acronyms

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We have pleasure in introducing the 8th edition of this publication, commonly called the SI Brochure, which defines and presents the Système International d'Unités, the SI (known in English as the International System of Units). This Brochure is published as a hard copy, and is also available in electronic form at

Since 1970, the Bureau International des Poids et Mesures, the BIPM (known in English as the International Bureau of Weights and Measures), has published seven previous editions of this document. Its main purpose is to define and promote the SI, which has been used around the world as the preferred language of science and technology since its adoption in 1948 through a Resolution of the 9th Conférence Générale des Poids et Mesures, the CGPM (known in English as the General Conference on Weights and Measures).

The SI is, of course, a living system which evolves, and which reflects current best measurement practice. This 8th edition therefore contains a number of changes since the previous edition. As before, it lists the definitions of all the base units, and all the Resolutions and Recommendations of the Conférence Générale des Poids et Mesures and the Comité International des Poids et Mesures, the CIPM (known in English as the International Committee for Weights and Measures), relating to the International System of Units. Formal reference to CGPM and CIPM decisions are to be found in the successive volumes of the Comptes Rendus of the CGPM (CR) and the Procès-Verbaux of the CIPM (PV); many of these are also listed in Metrologia. To simplify practical use of the system, the text provides explanations of these decisions, and the first chapter provides a general introduction to establishing a system of units and to the SI in particular. The definitions and the practical realizations of all the units are also considered in the context of general relativity. A brief discussion of units associated with biological quantities has been introduced for the first time.

Appendix 1 reproduces, in chronological order, all the decisions (Resolutions, Recommendations, Declarations) promulgated since 1889 by the CGPM and the CIPM on units of measurement and the International System of Units.

Appendix 2 exists only in the electronic version. It outlines the practical realization of some important units, consistent with the definitions given in the principal text, which metrological laboratories can make to realize physical units and to calibrate material standards and measuring instruments of the highest quality. This appendix will be updated regularly to reflect improvements in the experimental techniques for realizing the units.

Appendix 3 presents units used to measure actinic effects in biological materials.

The Comité Consultatif des Unités of the CIPM, the CCU (known in English as the Consultative Committee for Units), was responsible for drafting this document, and both the CCU and the CIPM approved the final text. This 8th edition is a revision of the 7th edition (1998); it takes into consideration decisions made by the CGPM and the CIPM since the 7th edition was published.

For more than thirty-five years this document has been used as a work of reference in many countries, organizations, and scientific unions. To make its contents accessible to a greater number of readers, the CIPM decided, in 1985, to include an English version of the text in the 5th edition; this double presentation is continued in all later editions. For the first English version the BIPM endeavoured to produce a faithful translation of the French original by close collaboration with the National Physical Laboratory (Teddington, United Kingdom) and the National Institute of Standards and Technology (Gaithersburg, United States), at that time the National Bureau of Standards. For the present edition the French and English versions were prepared by the CCU in close collaboration with the BIPM.

The 22nd CGPM decided, in 2003, following a decision of the CIPM in 1997, that "the symbol for the decimal marker shall be either the point on the line or the comma on the line". Following this decision, and following custom in the two languages, in this edition the point on the line is used as a decimal marker in the English text, and a comma on the line is used in the French text. This has no implication for the translation of the decimal marker into other languages. A point to note is that small spelling variations occur in the language of the English speaking countries (for instance, "metre" and "meter", "litre" and "liter"). In this respect, the English text presented here follows the International Standard ISO 31, Quantities and Units.

Readers should note that the official record is always that of the French text. This must be used when an authoritative reference is required or when there is doubt about the interpretation of the text.

March 2006

E. Göbel
I.M. Mills
A.J. Wallard
President, CIPM
President, CCU
Director, BIPM

Note: For the official text, please refer to the PDF files available at:
  • (in English) and
  • (in French).