International System of Units

The International System of Units (commonly abbreviated to SI from its French name Système Internationale d'Unités) is the international system of units of measurement established under the Metre Convention. It is based on seven "base units", with a potentially limitless number of "derived units".

The definitions of the base units and other fundamental aspects of the system are approved by the General Conference on Weights and Measures (CGPM), a diplomatic conference which meets every four years, based on recommendations from the International Committee for Weights and Measures (CIPM). The CIPM is also charged with agreeing many of the technical details of the system, such as the methods of realization of units and other details of conventional measurement techniques.

Base units

Unit name	Unit symbol	Physical quantity	Notes
metre	m	length
kilogram	kg	mass
second	s	time
ampere	A	electric current
kelvin	K	thermodynamic temperature
candela	cd	luminous intensity
mole	mol	amount of substance

The six original base units were selected in 1956 by the 9th CGPM (the mole was added in 1972). They represent a conventional choice: the choice made in the International System of Units is not the only possible choice of base units, nor is there any physical significance that the ampere was chosen as the base for electromagnetic units rather than, e.g., the coulomb.

The inclusion of the candela as a base unit is for both historical and practical reasons. There is an obvious technical need to classify many everyday light sources on the basis of their effect on the human eye. This is acheived by multiplying the radient power (measured in watts per steradian) by a standard luminosity function that models the response of the human eye: the result is the candela, which is also the base for a series of other luminosity units.

Multiples and submultiples

Multiples				Submultiples
Symbol	Prefix				Prefix	Symbol
da	deca	101		10−1	deci	d
h	hecto	102		10−2	centi	c
k	kilo	103		10−3	milli	m
M	mega	106		10−6	micro	µ
G	giga	109		10−9	nano	n
T	tera	1012		10−12	pico	p
P	peta	1015		10−15	femto	f
E	exa	1018		10−18	atto	a
Z	zetta	1021		10−21	zepto	z
Y	yotta	1024		10−24	yocto	y

External links

• International Bureau of Weights and Measures (BIPM)