Skip to main content
U.S. flag

An official website of the United States government

Dot gov

The .gov means it’s official.
Federal government websites often end in .gov or .mil. Before sharing sensitive information, make sure you’re on a federal government site.


The site is secure.
The https:// ensures that you are connecting to the official website and that any information you provide is encrypted and transmitted securely.

Guide for the Use of the International System of Units (SI)



Ambler Thompson, Barry N. Taylor


This 2007 edition of SP811 updates International System of Units (SI) usage as implemented in the 8th edition of the SI Brochure, incorporates a significant amount of additional material intended to answer frequently asked questions concerning the SI and SI usage, and updates the bibliography. Some changes in format have also been made in an attempt to improve the ease of use of the document.Because of the importance of the SI to both science and technology, NIST has over the years published documents to assist NIST authors and other users of the SI, especially to inform them of changes in the SI and in SI usage. For example, this third edition of the SP 811, Guide for the Use of the International System of Units (SI), replaces the second edition (1995) prepared by Barry N. Taylor which replaced the first edition (1991) prepared by Arthur O. McCoubrey. That edition, in turn, replaced NBS Letter Circular LC 1120 (1979), which was widely distributed in the United States and which was incorporated into the NBS Communications Manual for Scientific, Technical, and Public Information, a manual of instructions issued in 1980 for the preparation of technical publications at NBS.
Special Publication (NIST SP) - 811
Report Number
Created March 30, 2008, Updated November 10, 2018