calculating internal sub box volume

I see plenty of calculators and stuff online, but they mainly only work for the normal shaped boxes, or trapezoid designs. How do we calculate for the boxes that have the recessed area in the back in order to fit over the brace area that goes across the back of the cab?

 

break the cross sectional shape into multiple shapes. For example, triangles and squares, then volume is just the area of these shapes X the depth. Add or subtract the various shapes together as required and voila.

 

subtract out the volume of the recess...

 

Thanks...I had thought about that earlier and thought this might be the case but wasnt 100% sure.