Cubic Airspace Calculations

[baker11]

Can anybody confirm if my calculations for final box design are correct for the cubic feet?

13"x 13" x 5.5" x4"(bottom depth then top)

I inserted the dimensions into a volume calculator on bcae1.com and their cubic feet came out different than mine...

The box is being made out 1/2 mdf board.

From what I understand the calculations would go something like this

(5.5"-1/2"-1/2")+(4"-1/2"-1/2")=7.5"/2=3.75" for the average depth

So

(13-1/2-1/2)x(13-1/2-1/2)x3.75"=540"/1728=0.3125

any confirmation on this would be greatly appreciated, starting install tomorrow

Here is my 3D rendering:

[cracker]

just wondering if the 1/2" would stand up to the pressure of the subs. screws might pull out cuz the thinner wood and not alot of contact area for the glue to stick better. i just bring it up because my brother made a box outta 1/2 in and his 2 12s ,well while drivin down the road the bass was vanishing and thinkin they were burnin up, pulled over to see the pressure took the whole face off the sealed box, glued screws and only home to the old pioneers with theall black cone and gold logo on the dustcap. hey i was a teen back then and just gettin into it all those where the shit us lol