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

Background

Definitions of the SI base units and their historical context

International aspects of the SI

Unit conversions

