Air Space equation

It's basic geometry.  For a cube or prism (right square or right rectangle) the formula is

(Height X width X depth) = volume.  If it's a wedge or cylinder or other shape, you must use the proper mathematical formula.  There is a calculator on this site, look to the left and follow the links.

You can use whatever dimensions you like to achieve the desired volume, although there are some rules of thumb depending on what kind of speaker enclosure you require.  For subs, the ratio of one dimension to another is less important.  Also realize it is the inside volume that matters as this is the airspace that will interact with the speaker to create your sound, so if you measure the outside of an enclosure you must subtract the thickness of the walls from the measurements to end up with the inner dimensions.  To be accurate, you must also subtract the volume of anything inside the enclosure, such as braces, ports, the speakers themselves, etc.  Generally, if you are within about 10% for a sealed enclosure the performance change will be minimal.  For ported enclosures of any type the tolerance is about 1/2 (5%) to attain proper performance.

You want to be exact well go here https://www.the12volt.com/caraudio/boxcalcs.asp play around till you get what you want or you might try WINISD