The International System of Units is the form of the metric system that is generally in use around the world.
This document gives guidance for the use of this system. It is the primary American National Standard for
conformity to the International System of Units (or the SI, as it is abbreviated from its name in French,
Le
Système International d’Unités
).
Any measurable quantity can be represented in this system with the aid of just seven “base” units, used
directly for the quantities length, mass, time, electric current, temperature, amount of substance, and
luminous intensity, or by combinations (called “derived” units) of these seven. For example, the unit of
speed can be expressed by the unit of length divided by the unit of time. The SI is a complete and coherent
system.
This standard shows first the two classes of units (base and derived) that make up the SI, together with the
symbols by which they are known. Prefixes that allow the formation of decimal multiples and submultiples
are explained. Then come notes on the proper use of the SI units and symbols in many applications.
In the first Annex are lists of many units from non-SI systems with the appropriate SI units that should be
substituted and numerical conversion factors. Other annexes include rules for conversion and rounding, a
discussion of the advantages of SI units with definitions where appropriate, a history of the development of
the system, and a bibliography of source documents.