I've been meaning to do this for about a month now so it's about time.
Converting all your dimensions into inches and back to metric is messy, gives room for errors and is very time consuming. So don't do it! Just use metric. Here's how.
I have a wedge that has these internal dimensions
354mm high
350mm wide
159.5mm deep at the top
216mm deep at the bottom
I enter the dimensions expressed as decimetres into the CAA calculator like this
H 3.54
W 3.5
D 1.595
D 2.16
The result is 0.0135 which I multiply by 12^3 and the answer is
23.328 cubic decimetres = 23.328 Litres
Calculating the wedge properly you get 23.262 Litres so it's 0.066 Litres over which I think is accurate enough.
NOTE: Without the 0.5mm on the top depth it's less accurate.
The reason it works is because it's just a calculator and will give the volume using any unit but it divides the answer by 12^3. It's a pity they don't build sub boxes in cubic inches instead of cubic feet, then it would require no conversion. Also it may be interesting to know some history to help remember what a decimetre is.
In France in 1799 when the metric system was devised
It was decided that 1 cubic decimetre of water will be 1 Kg in mass and 1 Litre in volume
but later on they found it was inaccurate and made 1 Litre about 1.000000028 cubic decimetres. (something like that) but I think you will agree that for our purposes, 1 cubic decimetre is a Litre.
I have to thank Reefhog for pointing out my error. I was calling it "decametre" Can you imagine a sub enclosure that's 23 cubic decametres? Man that's about the size of a block of flats!
I find using the CAA enclosure calculator this way to be the easiest and fastest way to find the volume of a sub box and I hope it helps all you people out there too.
Doug