Theory:

All circles are similar to one another. So, the ratio of the circumference to that of diameter is a constant,
 
That is CircumferenceDiameter=constant(piπ)    
  
Therefore Cd= π (Where the approximate value of \(π\) is \(3.14\))  
 
\(C\)\(=\)  d · π  or \(C\)\(=\) d ·3.14
 
The diameter is twice the radius (\(2r\)), so the above equation can be
written as C=2r · π Note: \(d \)\(= \)\(2r\)
Therefore, the circumference of a circle, \(C = 2\)π\(r\) units. 
  
Circumference.svg