Skip to main content
U.S. flag

An official website of the United States government

Official websites use .gov
A .gov website belongs to an official government organization in the United States.

Secure .gov websites use HTTPS
A lock ( ) or https:// means you’ve safely connected to the .gov website. Share sensitive information only on official, secure websites.

NIST Offers U.S. Interpretations of Recent SI (Metric) Changes

SI globe

The National Institute of Standards and Technology (NIST) has issued a new American version of the English language SI Brochure, the eighth edition of the international standard reference guide to the modern metric system, the International System of Units (known as SI from the French "Le Systeme International d'Unites"). NIST is the U.S. technical representative to the International Bureau of Weights and Measures that defines the SI and coordinates the federal government policy on the conversion to SI by federal agencies.

SI plays an essential role in international commerce and is the common language of scientific and technological research and development. The 2008 edition of NIST's "The International System of Units (SI)" (Special Publication 330) covers recent changes in the system since 1998. It "Americanizes" the SI to cover correct U.S. usage of metric units, such as the accepted spelling of "meter" and "liter." In addition, the revised guide includes a new chapter on units for quantities that describe biological effects, and symbols for expressing values for enzyme catalytic activity in biology and medicine.

The U.S. Omnibus Trade and Competitiveness Act of 1988 designated "the metric system of measurement as the preferred system of weights and measures for United States trade and commerce." U.S. Public Law 110-69 in 2007, also known as the "American Competes Act," replaced the old definition of the metric system with the definition of the SI.

The 2008 editions of "The International System of Units", edited by Barry N. Taylor and Ambler Thompson, is available online along with additional information and measurement conversion tables. Hard copies can be requested by e-mail at TheSI [at] (TheSI[at]nist[dot]gov).

Released May 13, 2008, Updated February 6, 2023