The penis is actually measured from the top, and not along the underside as many men think.  If measured from the underside, the angle of an erection might influence the measurements. Measure your length from the tip of the head along the upper side of the shaft to the point where the penis joins the body. You can do this with a piece of string of a measuring stick.  For girth, measure the penis circumference at the widest point using a tape measure or a piece of string.