Theory:

A system of units is the complete set of units, both fundamental and derived, for all kinds of physical quantities.
The common systems of units are given below:
  
System
Length
Mass
Time
CGS
Centimeter
Gram
Second
MKS
Meter
Kilogram
Second
FPS
Feet
Pound
Second
 
Important!
In the system of units, the first three systems of units (CGS, SI, MKS systems) are the metric systems of units. FPS is not a metric system of units, it is a British system of units.
SI Units:
The SI system is also known as the international system of units and it is abbreviated from the French word 'systeme International'.
SI system of units is the modernized and improved form of the previous system of units.
 
It is accepted in almost all the countries of the world. It is based on a certain set of fundamental units from which derived units are obtained by multiplication or division.

There are seven fundamental units in the SI system of units, which given below:
  
S.No
Fundamental quantities
Unit
Symbol
1
Time
Seconds
s
2
Mass
Kilogram
kg
3
Length
Meter
m
4
Temperature
Kelvin
k
5
Luminous intensity
Candela
cd
6
Electric current
Ampere
A
7
Amount of substance
Mole
mol
 
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