We can measure length and girth, and not get a good understanding. I think I may have found a good estimation. Since an erect penis is a roughly cylindrical shape, we can measure it's volume with a simple formula. Height x Area of circle. Aread of circle = (pi)r^2 I am going to compare an average penis to one typically called large. Average: 6" long and 1.5" thick Large: 8" long and 2" thick Average volume: Height = 6", r = 1.5"/2 or 3/4" this equals 3 and 3/8(pi) cubic inches. Not bad Average volume: Height =8", r=2"/2 or 1" this equals 8(pi) cubic inches. whoa. It turns out that the large penis is more than twice the size of the average one. Plug in your own numbers to see how you measure up.