well B x W x H / 1728 = cu ft
those are internal dimensions.. If your porting it, u need to find the area of the port and take it away from the overall box volume, a port is displacement and isnt included in the airspace. As for designing, decide if yu want ported or sealed, and how big u want. After that u might need to make some crazy designs in the box, or deal with a sealed box instead of ported. Think of what u want it to look like, and want it to sound like, and then calculate a rough cu ft number, and work with that.
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