I need to know what mathematical formula is used to get 2 pvc measurements instead of using a measuring tool

pi times r squared... 3.14 time the radius times itself.
ex... radius is 5 inches, 5 times 5 = 25
pi = 3.14 times 25 = 78.50
[grinz]
now, what you going to do with it?

Could you specify what you are trying to measure?