SI

The International System of Units (abbreviated SI from the French phrase, Système International d'Unités) is the most widely used system of units. It is the most common system for everyday commerce in the world, and is almost universally used in the realm of science.

Units

SI consists of seven base units and several derived units, together with a set of prefixes.

Related Topics:
Base units - Derived units - Prefix

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Base units

The following are the fundamental units from which all others are derived, they are dimensionally independent. The definitions stated below are widely accepted.

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Dimensionless derived units

The following SI units are actually dimensionless ratios, formed by dividing two identical SI units. They are therefore considered by the BIPM to be derived. Formally, their SI unit is simply the number 1, but they are given these special names, for use whenever the lack of a unit might be confusing.

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Derived units with special names

Base units can be put together to derive units of measurement for other quantities. Some have been given names.

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Non-SI units accepted for use with SI

The following units are not SI units but are "accepted for use with the International System."

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SI prefixes

SI defines a number of SI prefixes to be used with the units: these combine with any unit name to give subdivisions and multiples. For example, the prefix kilo denotes a multiple of a thousand, so the kilometre is 1000 metres, the kilogram 1000 grams, and so on. The prefixes are never combined; a millionth of a kilogram is a milligram, and not a 'microkilogram'.

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The following SI prefixes can be used to prefix any of the above units to produce a multiple or submultiple of the original unit. This includes the degree Celsius (e.g. "1.2 m°C"); however, to avoid confusion, prefixes are not used with the time-related unit symbols min (minute), h (hour), d (day). They are not recommended for use with the angle-related symbols ° (degree), ' (minute of arc), and " (second of arc)http://physics.nist.gov/Pubs/SP811/sec06.html, but for astronomical usage, they are sometimes used with seconds of arc.

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Obsolete metric prefixes

The following metric prefixes are no longer in use: myria-, myrio-, and any double prefixes such as those formerly used in micromicrofarads, hectokilometres, millimicrons.

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See: Obsolete metric prefixes

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