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BIPM, International Bureau of Weights and Measures
(Bureau International des Poids et Mesures)
Serves as the "international" metrology institute and publishes the definitive international reference on the SI
IEC, International Electrotechnical Commission
Develops international standards for electrotechnology

ANSI, American National Standards Institute
Approves United States standards in many areas

ISO, International Organization for Standardization
Develops international standards in areas other than electrotechnology
NIST Metric Program
Seeks to accelerate the Nation's transition to the metric system

Information on the SI within this reference is primarily based on three NIST publications, which are available in electronic (acrobat pdf) format. (If you do not have this software, you may wish to obtain it free from Adobe.)

Note: The BIPM SI Brochure (9th Edition) was published on May 20, 2019. The corresponding editions of NIST SP 330:2019 is now available. An updated edition of SP 811 will be published soon that reflects the changes incorporated in the BIPM SI Brochure (9th Edition). Electronic copies of SP 330:2019 and SP 811:2008 are available online from the Bibliography.

Guide to the SI, with a focus on usage and unit conversions:
NIST Special Publication 811, 2008 Edition, by Ambler Thompson and Barry N. Taylor.
Guide for the Use of the International System of Units (SI)

    This publication, abbreviated SP 811, has been prepared by NIST to provide assistance in the use of the SI. The topics covered by SP 811 include:
      view publication
    • NIST policy on the use of the SI in NIST publications.
    • view publication Classes of SI units, those SI derived units that have special names and symbols, and the SI prefixes that are used to form decimal multiples and submultiples of SI units.
    • Those units outside the SI that may be used with the SI and those that may not.
    • order publicationRules and style conventions for printing and using quantity symbols, unit symbols, and prefix symbols, and for spelling unit names.
    • Rules and style conventions for expressing the results of measurements and the values of quantities.
    • Definitions of the SI base units.
    • Conversion factors for converting values of quantities expressed in units that are mainly unacceptable for use with the SI to values expressed mainly in units of the SI.
    • Rounding numbers and rounding converted numerical values of quantities.


Guide to the SI, with a focus on history:
NIST Special Publication 330, The International System of Units (SI), 2019 Edition, David B. Newell and Eite Tiesinga, Editors

    This publication, abbreviated SP 330, is the U.S. version of the English language text of the 8th edition of the brochure Le Système International view publication d'Unites (SI). The 8th edition of the brochure, which is the current definitive reference on the SI, was published in the French language in 2006 by the International Bureau of Weights and Measures (BIPM, Bureau International des Poids et Mesures). Except for very minor differences, SP 330 is identical to the English-language text that follows the official French language text in the two BIPM publications. order publication The BIPM SI Brochure, and thus SP 330, contains Resolutions and Recommendations of the General Conference on Weights and Measures (CGPM, Conférence Générale des Poids et Mesures) and the International Committee for Weights and Measures (CIPM, Comité International des Poids et Mesures) on the SI. Also included is explanatory material, as well as relevant extracts from the International Standards of the International Organization for Standardization (ISO) for the practical use of the SI.

Guide to the SI, with a legal focus:
Federal Register notice of May 16, 2008, 73 FR 28432-28433
Interpretation of the International System of Units for the United States

    This notice restates the interpretation of the SI for the United States by the Department of Commerce. view publication As provided by U.S. law, the metric system of measurement to be used in the United States is the SI as established by the General Conference on Weights and Measures and interpreted or modified for the United States by the Secretary of Commerce. The Secretary has delegated this authority to the Director of the National Institute of Standards and Technology.

Diagram of SI unit relationships:

    view publicationThis schematic diagram illustrates how the 22 SI derived units with special names and symbols are related to the seven SI base units. Versions optimized for printing are available.

History of the SI, with a focus on the BIPM
NBS Special Publication 420, 1975 Edition, C. Page and P. Vigoureux, Editors
The International Bureau of Weights and Measures 1875-1975

    view publicationThis publication, abbreviated SP 420, gives the history of the first century (1875-1975) of the International Bureau of Weights and Measures (BIPM, Bureau International des Poids et Mesures), including the history of the Convention du Mètre (Meter Convention) and the SI. It is out of print but is available as online PDF.

Weights and measurements in the United States, with a focus on history:
NBS Special Publication 447, 1976 Edition, Louis E. Barbrow and Lewis V. Judson
Weights and Measures Standards of the United States, a brief history

    view publication This publication, abbreviated SP 447, provides a brief history of U.S. weights and measures from the founding of the country to about 1975. It includes copies of the notices published in the Federal Register of the United States that give the relationships between various customary units of weights and measures and SI units. It is out of print but available online.

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Legal note: The Adobe Acrobat software is identified above in order to assist users of this information service. In no case does such identification imply recommendation or endorsement by the National Institute of Standards and Technology.

Online: March 1998   -   Last update: March 2020