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).
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.
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 unit of electrical current is the constant current which, if maintained in two straight parallel conductors, of infinite length and negligible cross-section, placed 1 metre apart in a vacuum, would produce a force between these conductors equal to 2×10 −7 newton per metre of length.
The unit of amount of substance is the amount of substance which contains as many elementary entities as there are atoms in 0.012 kilogram of pure carbon-12. [elementary entities may be atoms, molecules, ions, electrons, or particles].
The unit of luminous intensity is the luminous intensity, in a given direction, of a source that emits monochromaticradiation of frequency 540×1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.
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.