The NIST Reference on Constants, Units and Uncertainty Information at the foundation of modern science and technology from the Physical Measurement Laboratory of NIST

SI Units
Constants,
Units &
Uncertainty
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Note
The information on the SI below was current up until May 20, 2019 (World Metrology Day). Since then the SI has undergone major changes. Updated pages will soon become available.

In the meantime, users may refer to the Bureau International des Poids et Mesures (BIPM) SI Brochure (2019, 9th edition), NIST SP 330:2019, and the updated SI diagram. A future update of NIST SP 811:2008 is planned.


Essentials of the SI
    Introduction
    SI units and prefixes
    Units outside the SI
    Rules and style conventions

Background
    Definitions of the SI base units and their historical context
    International aspects of the SI
    Unit conversions

Bibliography
    Online publications and citations

 

 

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Formerly known as SRD 122, but reclassified as an information resource to be
consistent with the Standard Reference Data Act of 1968 and, as amended,
January 2017.

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Online: February 1998  -   Last update: March 2020