Length X Width X Height (In inches) = Cubic inches
Cubic inches Divided by 1728 will give you Cubic Feet
The above needs the Internal measurement of the chamber.
You are asking why 1728? 12 inches X 12 inches X 12 inches = 1728 That is how many Cubic inches are in a Cubic Foot
Yeah winISD works great, just enter the T/S parameters. A good thing to do if you can is enter one you have listened to and look at the curve, that will give you a better idea of what it will sound like. You can drag the box size and see it change, or port size. Yes it is good idea to read up on that stuff some so you get a basic understanding of what the perimeters mean....some subs are better for small sealed and some ported/etc.
I should add to that, because you get cabin gain that makes more bass than your sub does. So the curve on that software will not show it, but it will play lower than the curve for open space predicts. Depends on the vehicle and install, but maybe roughly 20Hz lower. Also your crossover will cut it off where you set it, so at 50Hz it would be say -3db from the chart and lower yet as you go higher.