Now measure from the middle of the surround on one side of your sub to the middle of the surround on the opposite side. That will give you your cone diameter. It will probably be around 6.5". Divide that in half to get the radius of the cone =3.25". Now use π × r2 = which gives you roughly 33 square inches for 1 8" sub. You are wanting to do 4 8" subs.... So you will need a line volume of 132 square inches.