The Metric System
officially called the
The metric system, or SI, is a decimal-based system of measurement used throughout the globe. In fact, several sources say it has been officially adopted as the system of measurement for every nation on Earth except Burma (Myanmar), Liberia and the United States.
The first metric system was introduced in France in 1799. It has evolved and several versions were introduced over the years until 1960, when the General Conference on Weights and Measures (CGPM) published today’s version and called it the International System of Units (SI). The SI has continued to be refined and revised since then, including the definition of a new base unit, the mole; definitions of additional derived units; the redefinition of several units and other refinements. The SI will further evolve in the future, as technology and science advances; allowing, for example, the units of kilogram, ampere, kelvin and mole to be redefined in terms of constants.
Structure of the Metric System
The SI consists of sets of base units, derived units and prefixes. The seven SI base units are the meter, kilogram, second, ampere, kelvin, mole and candela. The formal definitions of these base units are available on this web page.
are formed by combining the base units according to the algebraic relations
linking the corresponding quantities. There is no set number of derived units,
as the possibilities are endless. However, there are 22 units that have been
given special names for convenience of reference, as they are frequently used. For
example, the joule, symbol J, is by definition equal to m2 kg s-2. See the Metric Units page.
A set of 20 Prefixes have been officially established to express
multiples and sub-multiples of the base or derived units with special names. See the Metric Prefixes page.
Finally, there are several non-SI units that are accepted for use with the International System, because they are widely used in everyday life, are of historical importance or provide an advantage or convenience in a particular scientific, technological or commercial specialty. See the Metric Units page.
The defining document for the metric system is the brochure: International System of Units (SI) from the
The primary reference in the United States is the NIST Special Publication 330, 2008 Edition: The International System of Units (SI) from the National Institute of Standards and Technology.
Except for very minor differences, SP 330 is identical to the
English-language text of the BIPM Brochure. Those differences are: a.) the spelling of certain words in accordance with common U.S. practice (e.g., “meter,” “liter,” and “deka” instead of “metre,” “litre,” and “deca”); b.) using the term “metric ton” rather than "tonne" for that unit; c.) the addition of a few editor's notes and references; and d.) a few very minor editorial changes are made in order to “Americanize” some phrases.