I'm going to Explain what's Radius ,Diameter and pi.
Step 1: Formula and pi
The Rule is = pi X (R X R)
how to calculate or pi:
You can Calculate it by Drawing a Large Circle then measuring diameter and Curimefernce then divide the curimefrence by the Diameter
Step 2: Radius and diameter
Radius : the radius of a circle or sphere is the length of any such segment, which is half the diameter,the radius also refers to the distance from the center of a circle ,sphere cylinder...etc
the radius may be more than half the diameter (which is usually defined as the maximum distance between any two points of the figure).