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Alternate uses: see SI (disambiguation)
The International System of Units, abbreviated SI (for the French phrase Système International d'Unités), is the most widely used system of units. Along with the older cgs (centimetre, gram, second) system, SI is sometimes referred to as the metric system (especially in the United States, which has not widely adopted its use in everyday commerce, and the UK where conversion is incomplete).

Table of contents
1 Origin
2 SI writing style
3 Notes
4 Related Articles
5 External links


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.

SI is built on seven SI base units, such as the kilogram and metre. 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.

SI writing style

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. US, UK) define them in terms of SI units. 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.)


Americans frequently spell 'metre' as 'meter', and 'litre' as 'liter'; however 'metre' and 'litre' are the official BIPM names for these units, although the American usage has been approved by the US government. The official US spelling for 'deca' is 'deka', though Americans use the international spelling more often than the American one.

The unit 'gram' is also sometimes spelled 'gramme' in English speaking countries, though that is an older spelling. Several other languages use the American spelling. In written practice only the abbreviated (prefixed) symbols are used, avoiding the spelling issue.

Related Articles

External links