its always inner demensions, after all displacments are taking out --- so take the "outter inner" demensions (the biggest inner demensions) and find the volume, subtract the displament of the driver(s), braceing and ports -- the ports should be calculated as solid objects, not hollow, so your going to use the outside demensions of the ports -- in certain cases the above statments wont work, just be smart and logic your way through what messurments you need to be usuing. the first statment is always true however.