relation of pi with circles??

circumference=diameter*pi
area of a plane circle=pi*radius^2
volume of a sphere=(4/3)pi*radius^3
surface area of a sphere=4pi*radius^2

π is the ratio between the circumference and the diameter of a circle.

i.e. π = circumference / diameter

All circles carry the same π which is approximately 3.141592654...
or 22/7 (low accuracy).

Then, the following formulae of a circle follow :

circumference = diameter * π
___________= 2 * radius * π

area = radius * radius * π

I think the relation of pi with circles in two aspects as following:
Firstly, the perimeter of a circle is equal to two times pi and radius,
i.e. perimeter=2πr where r is the radius of a circle.
Secondly, the area of a circle is equal to pi times the square of radius,
i.e. area=πr^2 where r is the radius of a circle.
Blessings,
Samuel Lai Tze Cheong.