Originally Posted by horneyoldguy

The circumference of a circle can be found by multiplying pi ( π = 3.14 ) by the diameter of the circle.
If a circle has a diameter of 4, its circumference is 3.14*4=12.56
If you know the radius, the diameter is twice as large.
To find the diameter using the circumference, measure the circumference and divide by 3.14

In other words, measure around you cock and balls and divide by 3.14, then go to a hardware store and look for a rubber or metal (no nickel as stated in an early responce) for the size closest to what you need.