Just enter the INTERNAL dimensions of the box to find its volume, or you can enter the required volume and two of the dimensions to find the remaining one.
You can also work out the actual volume allowing for the speaker displacement.
If you have a wedge shapped box add the EXTERNAL top and bottom depths together and divide by 2, then multply the thickness of the MDF by 2 and deduct this from the first total.
Example:
If your box has a top depth of 10" and a bottom depth of 16" that total is 26, divide by 2 to get 13", the MDF is 1/2" thick, multiply by 2, and you get 1", 13" - 1" =12", use this figure to get your depth measurement.