description
 SI Base Quantities and Units
 Systems of Units and SI Units
notes
The International System Units
Internationally accepted system of units is Système Internationale d’ Unites (French for International system of Units) or SI. It was developed and recommended by General Conference on Weights and Measures in 1971. In earlier times, scientists of different countries were using different systems of units for measurements.
Systems of Units:
A system of units is the complete set of units, both fundamental and derived, for all kinds of physical quantities. The common system of units which is used in mechanics are given below:

CGS System: In this system, the unit of length is centimetre, the unit of mass is gram and the unit of time is second.

FPS System: In this system, the unit of length is foot, the unit of mass is pound and the unit of time is second.

MKS System: In this system, the unit of length is metre, the unit of mass is kilogram and the unit of time is second.

SI System: This system contain seven fundamental units and two supplementary fundamental units.
Base Quantity  Name  Symbol 
Length  metre  m 
Mass  kilogram  kg 
Time  second  s 
Electric Current  ampere  A 
Thermodynamic Temperature  kelvin  K 
Amount of Substance  mole  mol 
Luminous intensity  candela  cd 
Supplementary Fundamental Units
Radian and steradian are two supplementary fundamental units. It measures plane angle and solid angle respectively.
Some units retained for general use
Name  Symbol  Value in SI Unit 
minute  min  60 s 
hour  h  60 min = 3600 s 
day  d  24 h = 86400 s 
year  y  365.25 d = 3.156 × 10^{7} s 
degree  ^{o}  1^{o} = (`pi`/180) rad 
litre  L  1 dm^{3} = 10^{3} m^{3} 
tonne  t  10^{3} kg 
carat  e  200mg 
bar  bar  0.1 MPa = 10^{5} Pa 
curie  Ci  3.7 × 10^{10} s^{1} 
roentgan  R  2.58 × 10^{4} C/kg 
quintal  q  100kg 
barn  b  100 fm^{2} = 10^{28} m^{2} 
are  a  1 dam^{2} = 10^{2} m^{2} 
hectare  ha  1 hm^{2} = 10^{4} m^{2} 
standard atmospheric temperature  atm  101325 Pa = 1.013 10^{5} Pa 