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SI Brochure: The International System of Units (SI) [8th edition, 2006; updated in 2014]
Preface to the 8th edition

    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.

    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

We are pleased to present the updated (2014) 8th edition of the SI Brochure, which defines and presents the Système International d'Unités, the SI (known in English as the International System of Units).

Chapter 1: Introduction

Chapter 2: SI units

Chapter 3: Decimal multiples and submultiples of SI units

  • SI prefixes
  • Factor Name Symbol Factor Name Symbol
    101 deca da 10–1 deci d
    102 hecto h 10–2 centi c
    103 kilo k 10–3 milli m
    106 mega M 10–6 micro µ
    109 giga G 10–9 nano n
    1012 tera T 10–12 pico p
    1015 peta P 10–15 femto f
    1018 exa E 10–18 atto a
    1021 zetta Z 10–21 zepto z
    1024 yotta Y 10–24 yocto y
  • The kilogram

Chapter 4: Units outside the SI

Chapter 5: Writing unit symbols and names, and expressing the values of quantities

General principles for the writing of unit symbols and numbers were first given by the 9th CGPM (1948, Resolution 7). These were subsequently elaborated by ISO, IEC, and other international bodies. As a consequence, there now exists a general consensus on how unit symbols and names, including prefix symbols and names, as well as quantity symbols should be written and used, and how the values of quantities should be expressed. Compliance with these rules and style conventions, the most important of which are presented in this chapter, supports the readability of scientific and technical papers.

Appendix 1: Decisions of the CGPM and the CIPM

This appendix lists those decisions of the CGPM and the CIPM that bear directly upon definitions of the units of the SI, prefixes defined for use as part of the SI, and conventions for the writing of unit symbols and numbers. It is not a complete list of CGPM and CIPM decisions. For a complete list, reference must be made to the BIPM website, successive volumes of the Comptes Rendus des Séances de la Conférence Générale des Poids et Mesures (CR) and Procès-Verbaux des Séances du Comité International des Poids et Mesures (PV) or, for recent decisions, to Metrologia.

Since the SI is not a static convention, but evolves following developments in the science of measurement, some decisions have been abrogated or modified; others have been clarified by additions. In the SI Brochure, a number of notes have been added by the BIPM to make the text more understandable; they do not form part of the original text.

In the printed brochure, the decisions of the CGPM and CIPM are listed in strict chronological order in order to preserve the continuity with which they were taken. However in order to make it easy to locate decisions related to particular topics a table of contents is also provided, ordered by subject:

Appendix 2: Practical realization of the definitions of some important units

Appendix 3: Units for photochemical and photobiological quantities

Optical radiation is able to cause chemical changes in certain living or non-living materials: this property is called actinism, and radiation capable of causing such changes is referred to as actinic radiation. Actinic radiation has the fundamental characteristic that, at the molecular level, one photon interacts with one molecule to alter or break the molecule into new molecular species. It is therefore possible to define specific photochemical or photobiological quantities in terms of the result of optical radiation on the associated chemical or biological receptors.

In the field of metrology, the only photobiological quantity which has been formally defined for measurement in the SI is for the interaction of light with the human eye in vision. An SI base unit, the candela, has been defined for this important photobiological quantity. Several other photometric quantities with units derived from the candela have also been defined (such as the lumen and the lux, see Table 3 in Chapter 2).