Ive tried using the calculators online, but there is so many variables that come into play that idk how to go about it. I measured my current box and the area i want to use (This is a single sub facing back with the port back) and i want to do has subs up port back set up, so im not sure if that would change any of the measurements. but the height is 13 inches, the width is 42 inches, and the depth is 21 inches. I know i want to tune this box to 30 hz for nice quality. So basically i have 2 alpine SWR_12d4 that will be pushing 650 rms to each sub. If any other info is needed i can provide, i just am not good at building boxes