When he is fully erect, use a soft measuring tape (like one for measuring for clothing size) to do the measuring. Measure across the top of his penis from base of the penis to the tip. It's more accurate to make sure his penis is straight out from his body. You may have to push it down if he normally has an erection that points in an upward angle when he is erect or you may have to lift it up if it angles down when he is hard. Hope this helps.