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


The International System of Units, universally abbreviated SI (from the French Le Systéme International d´Unités), is the modern metric system of measurement. Long the dominant measurement system used in science, the SI is becoming the dominant measurement system used in international commerce.

The Omnibus Trade and Competitiveness Act of August 1988 [Public Law (PL) 100-418] changed the name of the National Bureau of Standards (NBS) to the National Institute of Standards and Technology (NIST) and gave to NIST the added task of helping U.S. industry increase its competitiveness in the global marketplace. It also recognized the rapidly expanding use of the SI by amending the Metric Conversion Act of 1975 (PL 94-168). In particular, section 5164 (Metric Usage) of PL 100-418 designates

the metric system of measurement as the preferred system of weights and measures for United States trade and commerce . . .

and requires that

each Federal agency, by a date certain and to the extent economically feasible by the end of fiscal year 1992, use the metric system of measurement in its procurements, grants, and other business-related activities, except to the extent that such use is impractical or is likely to cause significant inefficiencies or loss of markets for United States firms . . .

In January 1991, the Department of Commerce issued an addition to the Code of Federal Regulations entitled “Metric Conversion Policy for Federal Agencies,” 15 CFR 1170, which removes the voluntary aspect of the conversion to the SI for Federal agencies and gives in detail the policy for that conversion. Executive Order 12770, issued in July 1991, reinforces that policy by providing Presidential authority and direction for the use of the metric system of measurement by Federal agencies and departments.*

The Metric Act of 1866 allowed use of the metric system of measurement in the United States. In 2007, the 1866 law was amended by PL 110–69, also known as the America COMPETES Act. This amendment updated the definition of the metric system:

“The metric system of measurement shall be defined as the International System of Units as established in 1960, and subsequently maintained, by the General Conference of Weights and Measures, and as interpreted or modified for the United States by the Secretary of Commerce.”

The America COMPETES Act also repealed separate legislation on electrical and photometric units, as they are included in SI, and it established UTC (Coordinated Universal Time) as the basis for standard time in the United States.

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 Guide 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.

It is quite natural for NIST to publish documents on the use of the SI. First, NIST coordinates the Federal Government policy on the conversion to the SI by Federal agencies and on the use of the SI by U.S. industry and the public. Second, NIST provides official U.S. representation in the various international bodies established by the Meter Convention (Convention du Mètre, often called the Treaty of the Meter in the United States), which was signed in Paris in 1875 by seventeen countries, including the United States (51 countries are now members of the Convention).

One body created by the Meter Convention is the General Conference on Weights and Measures (CGPM, Conférence Générale des Poids et Mesures), a formal diplomatic organization.**  The International System was in fact established by the 11th CGPM in 1960, and it is the responsibility of the CGPM to ensure that the SI is widely disseminated and that it reflects the latest advances in science and technology.

This 2008 edition of the Guide corrects a small number of misprints in the 1995 edition, incorporates the modifications made to the SI by the CGPM and CIPM in the last 13 years, and updates the bibliography. Some minor changes in format have also been made in an attempt to improve the ease of use of the Guide.

In keeping with U.S. and International practice (see Sec. C.2), this Guide uses the dot on the line as the decimal marker. In addition this Guide utilizes the American spellings “meter,” “liter,” and “deka” rather than “metre,” “litre,” and “deca,” and the name “metric ton” rather than “tonne.”

March 2008

Ambler Thompson & Barry N. Taylor

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