I always took the width x length x depth to come up with sq inches. One hundred and forty four sq. inches equals a sq foot. Is that what you needed? If not let me know. Some people will tell you that the room behind the driver doesn't matter but, it does. I always tried to keep the depth at least 150% of the driver depth, from mounting ring to back of the magnet. That depth isn't written in stone though, it's just my personal goal when I start out. Depending on the place you have to set them up can cause a change in any dimension, but what you subtract from any dimension has to be added to another dimension and that could be confusing since the other two dimensions are different. If you'll remember that when you subtract a certain amount of sq. inches from one dimension it has to be made up for those sq. inches elsewhere.
I hope I haven't confused you even more. Year's ago I built subs to fit in some tight places in personal vehicles. I got Vance Dickerson's? The loudspeaker cook book and honed in on the info on subs. They are still publishing newer versions of The cook book so, you may want to get one. When I bought mine it was about $30. May have been some of the best money I ever spent.
One question. Is the total Q of the driver/drivers in the range of the type of box you'd like to build. If I recall correctly a driver that works in a sealed enclosure needs to be in a certain range of total Q. A driver spec'd for a ported sub will usually not be the best one for a sealed or open baffle application. If I recall correctly the total Q of a driver spec'd for a sealed enclosure is pretty close to the total Q of an open baffle application also. If you buy the cook book you won't have to depend on much from other's. The cook book will advise on what type of sound you like by a Q number. The lower the Q, this Q isn't the same total Q of the driver, the tighter the sub will sound., boomy-tight bass. I've built boxes that would give a Q of .3-1.5. I always liked a Q of .6-.7 for vehicles as the cabin gain would help give 8-10db of more output than it setting in open space. Remember though that the smaller the enclosure the more power you'll need and the low end output will suffer also. I generally like a home sub Q of apprx. .5-.6. I can't stand much bass from 80-130hertz, as those frequencies really bother me.
