SI units

SI units

Etymology: Fr, système international

the international units of physical amounts. Examples of these units are the mass of a kilogram, the length of a meter, and the precise amount of time in a second.

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SI units

the units of measurement generally accepted for all scientific and technical uses. Together they make up the International System of Units. The abbreviation SI, from the French Système International d'Unités, is used in all languages. There are seven base SI units, defined by specified physical measurements and two supplementary units. Units are derived for any other physical quantities by multiplication and division of the base and supplementary units. The derived units with special names are shown in Table 3.

SI is a coherent system. This means that units are always combined without conversion factors. The derived unit of velocity is the meter per second (m/s); the derived unit of volume is the cubic meter (m³). If you know that pressure is force per unit area, then you know that the SI unit of pressure (the pascal) is the unit of force divided by the unit of area and is therefore equal to 1 newton per square meter.

The metric prefixes can be attached to any unit in order to make a unit of a more convenient size. The symbol for the prefix is attached to the symbol for the unit, e.g. nanometer (nm) = 10⁻⁹m. The units of mass are specified in terms of the gram, e.g. microgram (μg) = 10⁻⁹kg.

Only one prefix is used with a unit. The use of units such as the millimicrometer is no longer acceptable. When a unit is raised to a power, the power applies to the prefix as well, e.g. a cubic millimeter (mm³) = 10⁻⁹m³. When a prefix is used with a ratio unit, it should be in the numerator rather than in the denominator, e.g. kilometers/second (km/s) rather than meters/millisecond (m/ms). Only prefixes denoting powers of 10³ are normally used. Hecto-, deka-, deci- and centi- are usually attached only to the metric system units, gram, meter and liter.

Owing to the force of tradition, one noncoherent unit, the liter, equal to 10⁻³ m³ or 1 dm³, is generally accepted for use with SI. The internationally accepted abbreviation for liter is the letter l; however, this can be confused with the numeral 1 in typescript. For this reason, the capital letter L is also sometimes used as a symbol for liter. The lower case letter is generally used with prefixes, e.g. dl, ml, fl. The symbols for all other SI units begin with a capital letter if the unit is named after a person and with a lower case letter otherwise. The name of a unit is never capitalized.