international system of units

The International System of Units (SI) is based on nine physical quantities. international system of units

Basic and supplementary units

Physical quantity	Unit name	Unit symbol
Length	metre	m
Mass	kilogram	kg
Time	second	s
Plane angle	radian	rad
Amount of substance	mole	mol
Electric current	ampere	A
Luminous intensity	candela	cd
Solid angle	steradian	sr
Thermodynamic temperature	kelvin	K

 

Derived units
By dimensionally appropriate multiplication and/or division of the units shown above, derived units are obtained. Some of these are given special names.

Physical quantity	Unit name	Unit symbol	Derivation
Electric capacitance	farad	F	(A2s4)/(kg m2)
Electric charge	coulomb	C	As
Electric conductance	siemens	S	(A2s3)/(kgm2)
Electric potential difference	volt	V	(kg m2)/(As3)
Electrical resistance	ohm	Ω	(kg m2)/(A2s3)
Energy	joule	J	(kg m2)/s2
Force	newton	N	(kg m)/s2
Frequency	hertz	Hz	1/s
Illuminance	lux	lx	(cd sr)/m2
Inductance	henry	H	(kg m2)/(A2s2)
Luminous flux	lumen	lm	cd sr
Magnetic flux	weber	Wb	(kg m2)/(As2)
Magnetic flux density	tesla	T	kg/(As2)
Power	watt	W	(kg m2)/s3
Pressure	pascal	Pa	kg/(m s2)

 

Some other derived units not having special names

Physical quantity	Unit	Unit symbol
Acceleration	metre per second squared	m/s2
Angular velocity	radian per second	rad/s
Area	square metre	m2
Current density	ampere per square metre	A/m2
Density	kilogram per cubic metre	kg/m3
Dynamic viscosity	pascal second	Pas
Electric charge density	coulomb per cubic metre	C/m3
Electric field strength	volt per metre	V/m
Energy density	joule per cubic metre	J/m3
Heat capacity	joule per kelvin	J/K
Heat flux density	watt per square metre	W/m2
Kinematic viscosity	square metre per second	m2/s
Luminance	candela per square metre	cd/m2
Magnetic field strength	ampere per metre	A/m
Moment of force	newton metre	N m
Permeability	henry per metre	H/m
Permittivity	farad per metre	F/m
Specific volume	cubic metre per kilogram	m3/kg
Surface tension	newton per metre	N/m
Thermal conductivity	watt per metre kelvin	W/(m K)
Velocity	metre per second	m/s
Volume	cubic metre	m3

 

Units: not SI
Some of the units which are not part of the SI system, but which are recognized for continued use with the SI system, are as shown.

Physical quantity	Unit name	Unit symbol	Definition
Angle	degree	°	(π/180) rad
Angle	minute	′	(π/10 800) rad
Angle	second	″	(π/648 000) rad
Celsius temperature	degree Celsius	°C	K – 273.2 (For K see 1.3.1)
Dynamic viscosity	poise	P	10-1Pas
Energy	calorie	cal	≈4.18J
Fahrenheit temperature	degree Fahrenheit	°F	9/5°C + 32
Force	kilogram force	kgf	≈9.807N
Kinematic viscosity	stokes	St	10¯4m2/s
Length	inch	in.	2.54 x 10‾2m
Length	micron	µm	10¯6m
Mass	pound	lb	≈0.454kg
Mass	tonne	t	103kg
Pressure	atmosphere	atm	101 325 Pa
Pressure	bar	bar	105Pa
Pressure	millimetre of mercury	mm Hg	≈133.322 Pa
Pressure	torr	torr	≈133.322 Pa
Thermodynamic temperature	degree Rankine	°R	°F + 459.7
Time	minute	min	60s
Time	hour	h	3600s
Time	day	d	86 400s

 

Decimal multiples of units
For quantities which are much larger or much smaller than the units so far given, decimal multiples of units are used.

Internationally agreed multiples are as shown.

For small quantities			For large quantities
Multiple	Prefix	Symbol	Multiple	Prefix	Symbol
10‾1	deci	d	10	deca	da
10¯2	centi	c	102	hecto	h
10¯3	milli	m	103	kilo	k
10‾6	micro	μ	106	mega	M
10¯9	nano	n	109	giga	G
10¯12	pico	p	1012	tera	T
10¯15	femto	f	1015	peta	P
10¯18	atto	a	1018	exa	E

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