How much space do i subtract from the overall internal volume of a sub-box ,for the displacement of the subs?

two 15 L7's will take out how much of the gross internal volume? for example i built the box to 8.8cubic feet if your suppose to measure from the port wall that is(not the full extent of the side wall right?) but how much space will i need to deduct for the subs to get a more accurate intern.size ...im trying to figure out my port length :)