Alright - first thing we gotta do is determine what airspace requirements and port size your subs need. Since you haven't told us what kind of subs they are, we dunno, and if you don't know I assume the subs don't come with a recomended box type or size. Please provide that info so we can help!
As to figuring volume - it was in teh initial post, but I suppose it's worth rehashing....
Multiply the
internal dimensions, Length x width x heght in inches. That'll give you the total number of cubic inches in the box. If you have a slant, average the bottom and top together - if it's 30 x 14 x 16 (bottom) and 12 (top), than it's 30 x 14 x 14, for example. Now, take your final cubic inch number and divide it by 1728, the number of cubic inches in a cubic foot. You now know the total raw capacity of the box in cubic feet. Adjust the measurements as necessary to get the airspace needed for your subs, and you're finished!!
When you get us the sub type and/or box requirements, we can get into the port size and whatnot, and make box size recomendations.
