🔊 Subwoofer Box Calculator
Work out the net internal volume for a sealed or ported subwoofer enclosure, the port length that tunes a ported box to your target frequency, and the external cabinet size from your panel thickness.
🔊 Enclosure spec
Net volume is the internal air space after bracing, port, and driver displacement. Cut the port to the length shown to hit your tuning frequency; verify against the sub manufacturer's recommended box specs.
Build the right box for your sub
A subwoofer only performs when the enclosure matches it. Too small and the bass turns thin and stiff; too big and it goes boomy and uncontrolled; and a ported box tuned to the wrong frequency loses the very output you built it for. Getting the net volume and — for a ported box — the vent length right is what turns a driver on paper into deep, clean bass in the car.
Enter your sub's recommended net volume, pick sealed or ported, and for a vented box add your tuning frequency and port diameter to get the port length from the vented-box equation. Pair it with the Amplifier Power and Speaker Impedance calculators to size the amp and wiring that will drive it.
❓ Frequently Asked Questions
How is the port (vent) length calculated?
It uses the standard vented-box equation Lv = (23562.5 × d² × Nports) / (Fb² × Vb) − (k × d), where Lv is the port length in centimetres, d is the port diameter in centimetres, Nports is the number of ports, Fb is the tuning frequency in hertz, Vb is the net internal volume in litres, and k ≈ 0.732 is the end-correction factor for a port free on one end. Larger diameter ports and lower tuning frequencies both need a longer vent.
What is the difference between a sealed and a ported box?
A sealed (acoustic-suspension) box is airtight and gives tighter, more accurate, more controlled bass with a smaller enclosure. A ported (bass-reflex) box adds a tuned vent that reinforces output around the tuning frequency for more low-end volume from the same power, at the cost of a bigger box and looser transient response. Match the choice to your sub's recommended enclosure specs.
Is the volume before or after driver and port displacement?
The figure you enter should be the net internal air volume the sub needs — the space left after subtracting the driver's own displacement, any bracing, and the port itself. Manufacturers publish this recommended net volume on the spec sheet. Build slightly larger and add fill or bracing to fine-tune rather than ending up short.
How are the external dimensions worked out?
The calculator converts your net internal volume to a cube, then adds two panel thicknesses to each edge to account for the walls on both sides of every axis. It is a starting point for a cube-shaped box; any shape with the same internal volume works, so adjust the height, width, and depth to fit your vehicle while keeping the internal volume constant.
What port diameter should I use?
Bigger ports move air with less turbulence (port noise or 'chuffing') but need to be longer to hit the same tuning. As a rule of thumb, use a larger diameter for higher-power subs, and add a second port rather than making one impractically long. If the calculated length is unwieldy, increase the diameter or the port count.