A subwoofer box calculator with port length and cut sheet
The right box size depends on the driver, not on its diameter. This subwoofer box calculator uses the driver’s Thiele/Small parameters (fs, Qts and Vas) to work out a sealed box for the Qtc you want, or a ported box with the classic QB3/B4 alignment, the tuning frequency and the port length. It then turns the volume into internal and outside dimensions and a cut sheet for your wall thickness.
A response plot compares the sealed and ported options so you can see the trade-off between box size, depth of bass and roll-off. Works in litres and millimetres or cubic feet and inches, entirely in your browser.
How to use the Subwoofer Box Calculator
- Choose Sealed or Ported and your units.
- Enter fs, Qts and Vas from the driver’s data sheet, and the driver displacement if listed.
- For sealed, set the target Qtc; for ported, keep the QB3/B4 alignment or enter your own volume and tuning, then the port size.
- Pick the wall thickness and box shape, then read the dimensions and the cut sheet.
Subwoofer Box Calculator features
Sealed alignment
Box volume, system resonance fc and f3 for any Qtc above the driver’s Qts.
Ported alignment
QB3 or B4 volume and tuning from Qts, including box losses, or check your own box.
Port length
Helmholtz formula with end correction for round or slot ports, one or more.
Dimensions
Golden-ratio, 1 : 1.26 : 1.59 or fixed height and width, inside and outside.
Cut sheet
Panel sizes for a butt-jointed box and the board area needed.
Formulas
- Sealed:
Vb = Vas ÷ ((Qtc ÷ Qts)² − 1)andfc = fs × Qtc ÷ Qts. At Qtc 0.707 the −3 dB point equals fc. - Ported: the box and port form a 4th-order high-pass filter (Thiele 1971, Small 1973). The QB3 family makes it as flat as possible for a driver with Qts below about 0.38–0.40; at the boundary it becomes the B4 (Butterworth) alignment: Vb = Vas ÷ 1.414, fb = fs and f3 = fs for a lossless box.
- Port:
L = c² × A ÷ (4π² × fb² × Vb) − k × D, where A is the total port area and k × D the end correction (0.732 D for a port flush with the baffle at one end and free inside the box).
Worked example
A driver with fs 30 Hz, Qts 0.38 and Vas 80 L, with box losses QL 7, gives a QB3 box of about 59 L tuned to about 32 Hz, with f3 near 34 Hz. A single 100 mm port needs to be about 32 cm long. The same driver sealed at Qtc 0.707 needs only 32.5 L, but its f3 rises to about 56 Hz. That is the usual trade: a ported box plays deeper for its size, a sealed box rolls off more gently and is more forgiving.
Check the amplifier power with the speaker wattage calculator and split frequencies with the crossover calculator.
Tips and limits
- Port air speed: a small port for a large, long-throw subwoofer can chuff. If in doubt, choose the largest port area that still fits, or a slot port along a wall.
- Ported boxes unload below the tuning frequency: use a subsonic filter to protect the driver.
- Data-sheet parameters vary between samples by 10–20%; measure them if you can.
- Stuffing, bracing and the driver’s own volume change the effective volume; the calculator adds driver displacement and the port, not bracing.
- The plot shows the response in open air. A car cabin adds strong low-frequency gain, so many car subwoofers are tuned higher.
Subwoofer Box Calculator FAQ
How do I calculate subwoofer box volume?
From the driver’s Thiele/Small parameters. For a sealed box Vb = Vas ÷ ((Qtc/Qts)² − 1); for a ported box use an alignment such as QB3. This subwoofer box calculator does both and adds the driver and port volume.
How long should my port be?
It depends on box volume, tuning and port area: L = c²A ÷ (4π²fb²Vb) minus the end correction. Bigger ports need to be longer for the same tuning.
Sealed or ported: which is better?
Ported boxes give more output near the tuning frequency and go deeper for their size; sealed boxes are smaller for low-Qts drivers, roll off gently and suit cars and music with tight bass.
What Qts is good for a ported box?
Roughly 0.2–0.4. Above about 0.4 a ported box tends to peak; sealed usually works better for high-Qts drivers.
What does the cut sheet include?
Two front/back panels, two sides and a top and bottom, sized for butt joints with your wall thickness. Double the baffle thickness if you want, and recheck the inside volume.
Does the box volume include the port and the driver?
The headline volume is the net air volume the driver sees. The gross internal volume and the dimensions add the driver displacement and the port.