Should be negligible. Call it .1 to .15 cubic feet and you'll be close enough that you would never notice the difference. Even really huge 80 pound magnet type subs only displace about .25 cube at most. and keep in mind how much of the cone and surround actually sets above the inside wall of your box. The internal volume the rest of it is taking up will rarely be an issue with which you need to super concern yourself.Thanks in advance. Cant find that number anywhere