Am I the only guy that's calculated this? I guess It's kind of hard to measure exactly...but if u look at the shaft as a cylinder and the head as a cone...then you can fairly good estimate. Or use water displacement....O_o. Well if you have or do after reading this feel free to share.9.69 cubic inches is my volume. (6 inches length, 5 inches girth)
I've always been curious about this myself. I'm just too lazy to actually try to measure the displacement of mine.
I imagine you could do it like Archimedes, if you fill some suitable container (like a vase or, in my case, an eye-cup) with water right to the brim, stick your penis in, and measure how much water overflows - most conveniently either by weighing the container before and after, or by measuring how much water is needed to fill it up again.