Subwoofer box calculator: sealed volume and port length

Sealed: enter the driver’s Fs, Qts and Vas and the net volume, and see Qtc, Fc and F3. Ported: enter the net volume, the tuning and the port size, and get the length to cut.

Which numbers you need

Three numbers from the driver’s datasheet do the box: Fs (free-air resonance), Qts (total Q) and Vas (the air volume as stiff as the suspension). Read the Vas unit twice: most US sheets print cubic feet, most European ones litres. If a sheet prints Qes and Qms but no Qts, Qts = Qes·Qms/(Qes + Qms). Not sure the numbers agree? The datasheet check tests them against each other.

The box volume here is the net volume: the air inside after the driver, the port, bracing and any double baffle are out. A box that measures 1.75 ft³ inside is about 1.55 ft³ net once a 12-inch driver and a 4-inch port are in.

Sealed: Qtc, Fc and F3

A sealed box adds the stiffness of its air to the driver’s suspension. With α = Vas/Vb, the box resonance is Fc = Fs·√(α + 1) and the box Q is Qtc = Qts·√(α + 1) (Small, 1972). A smaller box gives a higher Qtc and Fc.

0.577: the flattest group delay, tight, needs a bigger box.
0.707: maximally flat. The usual hi-fi target.
0.8 to 1.0: common in cars. Cabin gain lifts the low end, so a little punch at Fc works.
Above 1.1: boomy. The box is too small for that driver.

F3 is reported two ways because sites and datasheets use both: −3 dB below the flat line the response settles to, and, when there is a peak, −3 dB below the peak. The calculator shows which is which.

Ported: the port length

A port and the air in the box make a Helmholtz resonator. The acoustic length that tunes a net volume Vb to Fb with port area Sp is Leff = c²·Sp/(4π²·Fb²·Vb), with c = 343 m/s at 20 °C. The length you cut is shorter: the air just outside each end moves with the port, so each end adds an end correction.

This calculator, like the app, uses one flanged end (0.85 × radius, the end flush with the baffle) and one free end (0.61 × radius, the end inside the box). Our check against Hornresp found its implied correction at 1.46 to 1.62 × radius, and this default put the tuning within 1 % of it (see how we check the maths). A flared port or one that runs along a wall behaves as flanged at that end: choose it under Port ends.

Two warnings come from the same maths. If the port is longer than the box, fold it or use a smaller area. If its own pipe resonance, c/(2·Leff), falls below about four times your crossover, you may hear it as a note.

What this page leaves out

Port air speed and cone travel depend on the driver’s Sd, Xmax and your amplifier’s power. A port that tunes right can still chuff at 300 W. The app checks both at your amp’s real RMS and suggests the port area or the subsonic filter that fixes it. For the full response of a ported box, its F3 and the classic alignments (QB3, SBB4, SC4), use the app.

Sources

Results are design estimates. Check the driver’s datasheet and your amplifier’s minimum load before you build.

Checked against the sources: October 27, 2026