SI

For other uses, see SI (disambiguation).

The International System of Units, (symbol: SI) (for the French phrase Système International d'Unités), is the most widely used system of units. It is used for everyday commerce in virtually every country of the world except the United States. SI was selected from the existing Metre-Kilogram-Second system of units (MKS), with the addition of extra units, rather than the older Centimetre-Gram-Second system of units (CGS). SI is sometimes referred to as the metric system (especially in the United States, which has not widely adopted it, and the UK, where conversion is incomplete).

There are seven base units and several derived units, together with a set of prefixes. Non-SI units can be converted to SI units (or vice versa) according to the conversion of units.

Origin

The units of the SI system are decided by international conferences organised by the Bureau International des Poids et Mesures (International Office of Weights and Measures). The SI system was first given its name in 1960, and last added to in 1971.

Basis

SI is built on seven SI base units, such as the kilogram, metre and second. These are used to define various SI derived units.

SI also 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 1 000 metres, the kilogram 1 000 grams, and so on. Note that a millionth of a kilogram is a milligram, not a microkilogram.

SI writing style

• Symbols are written in lower case except for in symbols where the unit is the same as the name of a person, or derived from the name of a person. This means that the symbol for the SI unit for pressure, named after Blaise Pascal, is Pa, whereas the unit itself is written pascal. The official SI brochure lists the symbol for the litre as an allowed exception to the capitalization rules: either capital or lowercase L is acceptable.

• Symbols are written in singular e.g 25 kg (not "25 kgs")

• It is preferable to keep the symbol in upright roman type (for example, kg for kilograms, m for metres), so as to differentiate from mathematical and physical variables (for example, m for mass, l for length).

• A space is left between the numbers and the symbols: 2.21 kg, 7.3·10² m²

• SI uses spaces to separate decimal digits in sets of three. e.g. 1 000 000 or 342 142 (in contrast to the commas or dots used in other systems e.g. 1,000,000 or 1.000.000).

• SI used only a comma as the separator for decimal fractions until 1997. The number "twenty four and fifty one hundredths" would be written as "24,51". In 1997 the CIPM decided that the British full stop (the "dot on the line", or period) would be the decimal separator in text whose main language is English ("24.51"); the comma remains the decimal separator in all other languages.

The system can legally be used in every country in the world, and in many countries its use is obligatory. Those countries that still give official recognition to non-SI units (e.g. the US and UK) define them in terms of SI units; for example, the inch is defined to be exactly 0.0254 metres. It was adopted by the 11th General Conference on Weights and Measures (CGPM) in 1960. (See weights and measures for a history of the development of units of measurement.)

Units

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.

Dimensionless derived units

The following SI units are derived from the base units and are dimensionless.

Derived units with special names

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

The unit of volume litre, abbreviated L or l and being equal to 0.001 m³, is not an SI unit but is "accepted for use with the International System."

Spelling variations

Several nations, notably the United States, typically use the spellings 'meter' and 'liter' instead of 'metre' and 'litre', in keeping with standard American English spelling (for example, Americans also use 'center' rather than 'centre'; see also American and British English differences). In addition, the official US spelling for the SI prefix 'deca' is 'deka' (again, a variation not recognized by the BIPM).

The US government has approved these spellings for official use, but the BIPM only recognizes the British English spellings as official names for the units. In scientific contexts only the abbreviations are used; since these are universally the same, the differences do not arise in practice in scientific use.

The unit 'gram' is also sometimes spelled 'gramme' in English-speaking countries, though that is an older spelling and is falling out of use.

Related topics

• Other measurement systems:
  • Imperial units
  • U.S. customary units
  • Metre-tonne-second system of units
  • Chinese system of units
• Metrication
• Metric system in the United States
• Metrology
• UTC (Coordinated Universal Time)
• Binary Prefixes - used to quantify large amounts of computer data
• Orders of magnitude

External links

Official

• SI maintenance agency (home page)
• BIPM reference (SI reference)

Information

• US NIST reference on SI
• SI -- Its History and Use in Science and Industry
• Nelson, Robert A., "The International System of Units Its History and Use in Science and Industry". Via Satellite, February 2000.
• A Dictionary of Units of Measurement by Russ Rowlett

Further reading

• I. Mills, Tomislav Cvitas, Klaus Homann, Nikola Kallay, IUPAC: Quantities, Units and Symbols in Physical Chemistry, 2nd ed., Blackwell Science Inc 1993, ISBN 0632035838.