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
Units &
home page

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) website. Now that the 9th edition of the SI Brochure (2019) has been made available, we are in the process of revising SP 330, SP 811, and the contents of this site.

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

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

    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: June 2019