Classification of Units: a) Fundamental Units.
b) Derived Units.
c) Absolute and Gravitational Units.
a) Fundamental Units: The units of fundamental physical quantities (length, mass ,time) are known as fundamental units.
Fundamental Units are further subdivided into M.K.S., C.G.S., & F.P.S. Systems.
Out of them MKS & CGS are known as Metric Systems & FPS is known as British System.
Fundamental Physical Quantity Fundamental Unit
Length Metre m
Mass Kilogram kg
Time Second s
e.g.
Fundamental Units are further subdivided into M.K.S., C.G.S., & F.P.S. Systems.
Out of them MKS & CGS are known as Metric Systems & FPS is known as British System.
Length Mass Time
Metric systems.
M.K.S. System.
m
kg
s
C.G.S.System.
cm
g
s
British Imperial system.
F.P.S. System.
foot
pound
s
Further examples of primary/fundamentals units are : ampere, Kelvin, candela, mole.
Supplementary units: rad (RadianFor measurement of Plane angle).
steradian (SteradianFor measurement of Solid angle ).
LENGTH :
1 inch = 2.54 cm = 25.4 mm
1 cm = 0.3937 inch,
1mm = 0.03937inch
1 ft = 30.48 cm = 0.3048 m
1 yard = 91.44 cm = 0.9144m
1 m = 39.37 inch
= 3.281 ft
= 1.094 yard
= 100 cm
1 micron = mm = 0.001mm = 10^{3} mm
1 ft = 12 inch
1 yard = 3 ft
22 yard = 1 chain
220 yard = 1 furlong = 10chain
8 furlong = 1 mile
1 kilometre = 3281 ft = 1094 yard = 0.622 mile = 1000 m =
1000 x 10^{6} micron = 10^{9} micron
1 mile = 5280 ft = 1760 yard = 1.609 km = 1609 x 10^{6}
micron
METRIC SYSTEM (M.K.S.)

10 mm = 1 cm

10 cm = 1 dm (deci metre)

10 dm = 1 m

10 m = 1 dam (deca metre)

10 dam = 1 hm (hector metre)

10 hm = 1 km (kilo metre)

AREA :
1 sq. Inch = 6.4516
sq cm
1 sq. cm = 0.155 sq inch,
1 sq. mm = 0.00155 sq inch
1 sq ft = 0.0929 sq m
1 sq yard = 0.8361 sq m
1 sq m = 10.765 sq ft
= 1.1968
sq yard
=10,000
sq cm
1 are = 100 sq m
1 acre = 4840 sq yard = 4047 sq m = 40.47 are = 0.4047
hectare
1 hectare = 2.4711 acre = 100 are
1 sq km = 100 hecatre
1 sq mile = 259 hectare
1 sq ft = 144 sq inch
METRIC SYSTEM (M.K.S.) 
100 sq mm = 1 sq cm or 1 cm^{2} 
100 sq cm = 1 sq dm or 1 dm^{2} (deci metre square) 
100 sq dm = 1 sq m or 1 m^{2} 
100 sq m = 1 sq dam 
100 sq dam = 1 sq hm or 1 hm^{2} (hecto metre square 
VOLUME :
1 cu Inch =1 6.4 sq cc
(cc means cubic centi meter)
1 sq. cm = 0.061 cu inch,
1 cu mm = 0.000061 cu inch
1 cu ft = 6.23 gallon = 28.317 litre = 0.028317 cu m
1 cu yard = 0.76455 cu m
1 cu m = 35.3198 cu ft
= 1.3093 cu
yard
=1000
litre
=10,0000 cu
cm = 10^{6} cu cm
1lite  0.22 gallon =
1000 ml
1 gallon = 4.546 litre
1 pint = gallon = 0.56825 litre = 568.25 ml
WEIGHT :
1 gram = ounce
1 ounce = 28.35 g