Base units, Derived units and Non-SI units.
The 24 prefixes approved for use with SI units.
Factors for converting between SI units and some non-SI units commonly used in the U.S.
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. (Not true, as the U.S. has officially adopted the SI. It just hasn't been widely forced upon the public at large. But I digress.)
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.
On November 16, 2018, in Versailles, France, representatives from of 60 countries unanimously voted to revise the SI so that does not depend on physical objects. Instead, it’s based entirely on the speed of light and six other “constants” of nature. The revised system formally took effect May 20, 2019.
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.
Derived units 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 24 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 International Bureau of Weights and Measures (BIPM). The BIPM is based in Sèvres, in the suburbs south-west of Paris. The brochure, and other BIPM documents, are available in French, English and other languages. The authoritative reference is the French version.
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. However, per the NIST website, since the release of the 2019 edition of the BIPM document. "... the SI has undergone major changes. Updated pages will soon become available" for the NIST publication.
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.