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Everything You Want To Know About SI (But Were Afraid to Ask)

Relax, even scientists can sometimes use help when making conversions and measurements with the modern metric system, the International System of Units (known as SI from the French "Le Systeme International d'Unites".) The good news is that the National Institute of Standards and Technology (NIST) has just released a guide for researchers, available to all, on correct SI usage and unit conversion.

NIST Special Publication (SP) 811, Guide for the Use of the International System of Units, complements the recently released U.S. version of the English language SI Brochure, SP 330, the eighth version of international standard reference guide to the modern metric system. (See "NIST Offers U.S. Interpretations of Recent SI (Metric) Changes".)

SP 811 offers an extensive conversion factor appendix useful for measurement unit conversions and appropriate rounding strategies for data. It also provides an editorial checklist for reviewing manuscripts' conformity with the SI and the basic principles of physical quantities and units. A color chart has been added that illustrates the utilization of the SI base units in defining the 22 derived units with special names and symbols.

NIST SP 811 and NIST SP 330 are available online at and at Printed copies can be requested with an email to TheSI [at] (TheSI[at]nist[dot]gov).

Released June 10, 2008, Updated January 23, 2023