Enter a box’s outside dimensions to get its net air space, the number to compare with your subwoofer’s recommended box volume.
Gross inside–
Net air space–
Net (liters)–
Enter the outside width, height and depth.
How the calculation works
Each inside dimension is the outside dimension minus two wall thicknesses. A doubled front baffle takes one more thickness off the depth. Multiply the three inside dimensions to get cubic inches and divide by 1,728 for cubic feet. That is the gross volume.
Net volume is what is left for the air after everything inside the box takes its share: the subwoofer itself (its displacement, listed on most spec sheets), the port on a ported box, and any bracing.
A worked example
A sealed box 26 x 14 x 14 inches outside, built from 3/4-inch board, is 24.5 x 12.5 x 12.5 inches inside: 3,828 cubic inches, or 2.22 cubic feet gross. With one subwoofer displacing 0.10 cubic feet, the net volume is about 2.12 cubic feet.
What to compare it with
Your subwoofer’s spec sheet lists a recommended sealed and, often, ported volume, sometimes as a range. A box well outside that range will not sound the way the maker intended, however well it fits the vehicle. Shallow-mount subwoofers tend to want smaller boxes; large high-excursion subwoofers often want much larger ones. Use the figure for your model, not a general rule for the size.
