Are those internal or external?? 7th grade geometry tells me that Height * Width * Length = Volume -yes you need to use all the same units, this will yield cubic "units" -you quoted inches, to change cubic inches to cubic feet divide the total by 1728 (12 cubed) And for a box you HAVE to use the internal dimensions. Then you need to subtract the portion of the box that the port is blocking AND subtract for the volume that the sub displaces. Yea, see when I did that I found that info and I assumed it was wrong because I took external dimensions.... Thanks... Also thank you to the fellas above... I will look into that amp...