Good question:

If you use geometry to find this kind of volume as a lot of people on here do, you know are risking being way off. A penis is not a cylinder, and it is definiatly not a rectangular solid. (You need to at least use pi!)

Algebra would be better, you are right. Give me a formula, let's see how it works.

Calculus on the other hand is best. I bet your Algebra formula is Calculus even if you didn't realize it. Finding the volume of a penis is like, say finding the volume of an irregular shaped lake. Make sense?