internal LxWxH/1728= cu ft easy
external L(minus 1.5 if 3/4 mdf is used(or plywood in KY) x W(minus 1.5) x H (minus 1.5) / 1728 = cu ft
or if its a wedge
internal top width(depth) + bottom width / 2= average width then L x W x H / 1728 = cu ft
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