SI units

The International System of Units (abbreviated SI from French: Système international d'unités) is the modern form of the metric system. It comprises a system of units of measurementdevised around seven base units and the convenience of the number ten. The SI was established in 1960, based on the metre-kilogram-second system, rather than the centimetre-gram-secondsystem, which, in turn, had several variants. The SI has been declared to be an evolving system; thus prefixes and units are created and unit definitions are modified through international agreement as the technology of measurement progresses, and as the precision of measurements improves.

SI is the world's most widely used system of measurement, used in both everyday commerce and science.The system has been nearly globally adopted with Burma, Liberia and the United States not having adopted SI units as their official system of weights and measures. While only the US does not commonly use metric units outside of science, medicine, and the government, the United Kingdom has officially adopted a partial metrication policy, with no intention of replacing imperial units entirely. Canada has adopted it for most purposes but imperial units, which are used in the United States, are still legally permitted and remain in common use throughout a few sectors of Canadian society, particularly in the buildings trades and railways sectors.

Units and prefixes

SI base units^[13][14]

Unit name	Unit symbol	Quantity name	Quantity symbol	Dimension symbol
metre	m	length	l (a lowercase L), x, r	L
kilogram [note 1]	kg	mass	m	M
second	s	time	t	T
ampere	A	electric current	I (an uppercase i)	I
kelvin	K	thermodynamic temperature	T	Θ
candela	cd	luminous intensity	Iv (an uppercase i with lowercase non-italicized v subscript)	J
mole	mol	amount of substance	n	N
Note ^ Despite the prefix "kilo-", the kilogram is the base unit of mass, the kilogram, not the gram, is used in the definitions of derived units. Nonetheless, units of mass are named as if the gram were the base unit.