[Pee Wee Herman]

It has to be on top of, because the only accurate measure of penis length is from your stomach to the tip.

And what's the circumference. Measure it by wrapping a piece of string once around your unit, mark the string with your finger, take string off, and measure string. using pi to calculate the diameter isn't precise because your stinky pinky isn't a perfect circle, but you can get a rough idea of the diameter by using pi.