The International System of Units

The International System of Units (SI, abbreviated from the French Système international (d’unités)) is based on the metric system and used throughout the sciences. The intergovernmental organization Bureau international des poids et mesures (BIPM) has the mandate, i.e., to maintain this single, coherent system of measurements throughout the world.

It is defined by the seven SI base units and, since 20 May 2019, on seven exact universal constants that describe the natural world.

Resources:

• Wikipedia: 2019 redefinition of the SI base units
• American Physical Society: Living with the new SI (2019 March 25)
• National Institute of Standards and Technology:
  • Turning Point for Humanity: Redefining the World’s Measurement System on the NIST website with a lot of background information, images, and explanatory videos, last updated on November 2020 November 30.
  • Anything else you would like to know about Physical Constants on the NIST website.

• Bureau International des Poids et Mesures:
  • Weblink of the 9th edition of the SI Brochure (2019), which defines and presents the Système International d’Unités, the SI (known in English as the International System of Units).
  • PDF file of a short, 5-page concise summary of the SI system
  • PDF file of the complete, long, 100-page English 9th edition of the SI Brochure (2019) with every information you need/would like to know about the SI system.
  • PDF file of a 2-page check list for the proper use of quantities and units.