SI (Systeme International) system of units

The International System of Units or SI system abbreviated from its french name Systeme International is a system of units adopted by most of the world as an international standard of units.

With the explosion of discoveries and research happening in various fields during 20^th century, the need for a globally accepted system of units was seriously felt. This was necessary for coordination between hundreds of different disciplines, regions and organizations spread throughout the world. The General Conference on Weights and Measures (CGPM) after much hard work published SI system of units in 1960. SI system is recognized and adopted by most of the developed world today.

Base units

There are seven base units in SI system. All other units are derived from these:

Name	Symbol	Quantity
metre	m	length
kilogram	kg	mass
second	s	time
ampere	A	electric current
kelvin	K	thermodynamic temperature
mole	mol	amount of substance
candela	cd	luminous intensity

Derived units

Derived units are units that are derived from base units. Derived units do not always have special names for example unit of area is metre square but SI system recognizes 22 derived units with special names:

Name	Symbol	Quantity
hertz	Hz	Frequency
radian	rad	Angle
steradian	sr	Solid angle
newton	N	force, weight
pascal	Pa	pressure, stress
joule	J	energy, work, heat
watt	W	power
coulomb	C	electric charge
volt	V	voltage
farad	F	capacitance
ohm	Ω	resistance
siemens	S	electrical conductance
weber	Wb	magnetic flux
tesla	T	magnetic flux density
henry	H	electrical inductance
degree Celsius	°C	temperature
lumen	lm	luminous flux
lux	lx	illuminance
becquerel	Bq	radioactivity (decays per unit time)
gray	Gy	absorbed dose (of ionizing radiation)
sievert	Sv	equivalent dose (of ionizing radiation)
katal	kat	catalytic activity