Subwoofer Box Calculator

Works out the net internal volume of a subwoofer box from its outside dimensions, and the port length that tunes a vented one to a chosen frequency. Enter the outside width, height and depth, the panel thickness and whatever displaces air inside, and the volume comes back in cubic inches, cubic feet and litres. Add the driver's Vas, Qts and Fs and the sealed alignment is named, with the system Q, the box resonance and the minus three decibel point that follow from them. The volume and the tuning a driver actually wants are set by its own Thiele-Small parameters, not by the box, so this builds the box you specify rather than choosing it for you.

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Measurements In
Every dimension, displacement and volume on this form follows this choice, including Vas, and so does every figure in the answer. Frequencies and Qts are the same number in both.

Outside measurements in, net internal volume out. Add the driver Vas, Qts and Fs and the sealed alignment is named as well.

3/4 in MDF is 0.75; the metric sheet it sits beside is usually 18 mm, which is not the same board. Measure it rather than trusting the label; sheet goods often run under. Left blank it assumes 3/4 in.
The air the motor and basket take up behind the baffle, from the driver spec sheet. A 12 in sub is often 100 to 150 cu in, which is 1.6 to 2.5 litres.
Braces, the terminal cup and any port that ends up inside the box.
Leave Vas, Qts and Fs at zero to get the volume only. Pick millimetres and litres above and Vas goes in as the litre figure the spec sheet prints, with no conversion to do.
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Measurements In
The net volume and all three port dimensions follow this choice, and so does the port length that comes back. The tuning frequency is hertz either way.

Give the net volume and the tuning you want and this returns the port length. The volume and the tuning themselves come from the driver.

After the driver and the bracing are subtracted. The volume action works it out from the outside dimensions.
Where the box resonates and the port takes over from the cone. It follows from the driver and the alignment you are building, not from the box.
All the same size. They are tuned as one combined area and each is cut to the same length.
Inside, not outside. Leave at zero for a slot port.
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