Your calculation will not give volume which is the cross sectional area of the penis times its length. Since we rarely have a perfectly round cylindrical penis with flat ends we can make an estimate. Get a measurement across the dick and measurement from top to bottom somewhere along the length and average those two measurements. Then take half of that number (radius) and multiply it by itself and then multiply that by 3.14 (pi) and then by length, all in same units and you will get cubic inches or cubic centimeters or cubic yards or cubic miles depending on your size and most appropriate final units.