A Guide to SI Units and Their Usage

Physical quantities like weights and measures have been standardized through a network of international agreements which collectively form the SI or International System of Units (with the French name Système International d’Unités).

The system is maintained by the International Bureau of Weights and Measures in Paris, and is updated every few years.

SI base units and symbols
 Quantity  Unit  Symbol
 Length  meter  m
 Mass  kilogram  kg
 Time  second  s
 Electric Current  ampere  A
 Temperature  kelvin  K
 Amount of substance  mole  mol
 Luminous intensity  candela  cd

SI consists of base units which are absolute and independent of each other.

The base units are consistent with the metric system (referred to as the MKS system), with the acronym indicating the first letters of the symbols associated with the units of measurement of length, mass and time.

The physical quantities, associated SI units and symbols are illustrated in the adjoining table.

In addition to the base units, there are two types of units derived from the base ones, those with and without special names associated with them.

The derived units and the relationship between them can be found here.

Specific prefixes are associated with different factors corresponding to the factor of the SI unit, like kilo for thousand, mega for million and giga corresponding to billion.

The kilogram is the only SI unit which has the prefix built into the unit.

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