See also:

The International System of Units (Système international d'unités, abbreviated SI in all languages), which is in use in most European countries, has been adopted by Canada, and is popularly called the metric system. This system of weights, measures, and physical quantities is decimal throughout.

The SI units are of three kinds: base, supplementary, and derived. There are seven base units, one for each physical quantity, and two supplementary units (Table 1). Derived units are obtained by the multiplication and division of base and supplementary units. Several of these derived units have been given special names and symbols (Table 2), and can be expressed in terms of other units and base units (Table 3). Table 4 gives examples of SI prefixes and symbols. Several conversion factors are listed in Table 5. Some units continue to be used with SI (Table 6), whereas other units should not be used (Table 7).

1. The symbols are always in upright type, usually roman type.
   
   
2. Symbols remain unaltered in the plural.
   
   
3. Symbols are written without a period, except at the end of a sentence.
   
   
4. When the symbol for a unit comprises letters, a full space is left between the number and symbol, e.g. 45 kg, except when the first character of a symbol is not a letter, e.g. 32°C.
   
   
5. Symbols for SI units should always be used; unit names should not be written out except in general terms such as several metres west. Names and symbols should not be mixed.
   
   
6. In North America, a period or dot is used as the decimal marker, and should be positioned in line with the base of the numerals. Outside of North America and in French text, the comma is used in place of the period.
   
   
7. In numbers with many digits, the numbers are broken into readable blocks of three digits each starting from the right and left of the decimal point, e.g. 1 000 000. No space is left in a four-digit number (e.g. 1000) except for uniformity where four-digit numbers occur in tables.
   
   
8. When a decimal fraction is used, a zero should always be placed to the left of the decimal marker, e.g. 0.78 g.