Do you have more than one subwoofer?

.50 cu ft X # of subwoofers

2 subs would need a box that was 1 cu ft. This isn't a 1x1x1 box, it'll larger than these dimensions. Internal volume is not figured using the external measurements. A "1 cu ft box" also must be larger to account for the space displaced by the subwoofer magnets.

http://www.crutchfield.com/S-PCvv0Qw...&avf=N&search=