Here, I'll even do a step by step with real world numbers.
-I have a pair of 10" AA Assassins that I want to put in a 4ft^3 box tuned to 30hz.
-I have already determined that I'll be using a 14x48" face with whatever depth I need to get the volume I need.
-I'll use a side firing 8" aeroport to make this easier without having to deal with a slot port with turns in it.
-Plugging the 4ft^3 figure and 30hz tuning into the carstereo.com calc, I get a hair under 32" port length. This is equal to slightly under 1ft^3 port displacement, so let's say port/sub displacement is about 1.25ft^3.
-So now I know that my gross box volume needs to be roughly 5.25ft^3, so I plug my 48x14 figures into carstereo.com's box volume calculator, along with my desired gross volume(5.25ft^3) and wood thickness(.75"), and find out that I need the box to be 17-1/8" deep to have a 5.25ft^3 gross volume, which after port and sub displacement, will give me 4ft^3 tuned to 30hz.