How to calculate subwoofer box volume (net vs gross)

To calculate subwoofer box volume, subtract twice the panel thickness from each external dimension, multiply the three internal dimensions, then subtract the driver displacement, the port and the bracing. What is left is the net volume, which is the number your driver's recommendations and every box formula refer to.

Net vs gross volume

Two volumes matter, and confusing them is the most common box-building mistake:

• Gross internal volume is the empty air space inside the panels, before anything is mounted.
• Net volume is what remains for the driver to work against: gross minus driver displacement, port volume and bracing.

The external size of the box (outside dimensions multiplied together) is a third number. It only matters for whether the box fits in your trunk, never for acoustics.

Step-by-step calculation

1. Measure the external dimensions (width, height, depth).
2. Subtract the panel thickness twice from each one. With 18 mm (3/4") MDF that is 3.6 cm (1.5") per dimension, assuming panels on all six sides.
3. Multiply the internal dimensions. In centimetres, divide cm³ by 1000 to get litres. In inches, divide in³ by 1728 to get ft³.
4. Subtract the driver displacement from the spec sheet.
5. Subtract the port: the air inside it and the wood that forms it.
6. Subtract bracing and anything else inside: terminal cups, a double front baffle, wedges.

Worked example: 70 × 40 × 45 cm box in 18 mm MDF

Here is a real calculation for a single 12" ported box. External dimensions are 70 × 40 × 45 cm (27.56 × 15.75 × 17.72 in), built from 18 mm MDF on all sides.

1. Internal dimensions and gross volume

Gross internal volume = 66.4 × 36.4 × 41.4 = 100,062 cm³ = 100.06 L (3.53 ft³). For comparison, the external volume is 126 L (4.45 ft³), so the wood alone takes up about a fifth of the box.

2. Driver displacement

Our example driver's spec sheet lists 4.0 L (0.14 ft³) displacement. If yours does not, measure the motor and basket roughly as cylinders, or use 2-5 L for a typical 12".

3. Port volume

The box uses a slot port running front to back along one side: an opening 6 cm wide by the full internal height of 36.4 cm, 38 cm long, formed by a single 18 mm MDF divider. The box wall provides the other three sides.

• Air in the port: 6 × 36.4 × 38 = 8,299 cm³ = 8.30 L
• Divider panel: 1.8 × 36.4 × 38 = 2,490 cm³ = 2.49 L
• Total port volume: 10.79 L (0.38 ft³)

Count the port's air as well as its walls. The air inside the port moves as a mass and does not act as part of the box's spring.

4. Bracing

Two window braces cut from 18 mm MDF add up to about 1.0 L (0.04 ft³). Estimate bracing by multiplying each brace's actual wood dimensions; ignore the holes you cut out.

5. Net volume

With a 218 cm² slot (6 × 36.4 cm) and a 38 cm physical length, this box tunes to about 39 Hz using the Helmholtz formula with a flanged inner end and a free outer end. The port area works out to 2.6 cm² per litre, just under the usual rule-of-thumb range of 2.7-3.6 cm² per litre, so it is worth checking peak air speed at full power. The port length guide walks through that maths in detail.

Getting displacement right

Driver displacement depends on mounting. A standard front-mounted sub only displaces air with its basket and motor. An inverted (magnet-out) driver puts its cone volume inside the box instead, which is usually smaller. Manufacturers usually quote standard mounting. Two drivers mean twice the displacement, so do not forget it in dual-sub boxes.

How much do errors matter?

A sealed box is forgiving: being 10% off changes Qtc only slightly. A ported box is more sensitive because the tuning frequency depends on net volume. Since Fb scales with 1/sqrt(Vb), a 10% volume error shifts tuning by about 5%, which is roughly 1.5-2 Hz around 33 Hz. That is audible but rarely a disaster. Forgetting a 10 L port in an 84 L box, however, is a 13% error, and forgetting it in a smaller box is much worse.

Doing it in inches

The process is the same. A box measuring 27.56 × 15.75 × 17.72 in with 0.71 in (18 mm) panels has internal dimensions of 26.14 × 14.33 × 16.30 in. Multiplied, that is 6,106 in³, and 6,106 / 1728 = 3.53 ft³. Mixing units is where most mistakes happen, so pick one system and convert only at the end: 1 ft³ = 28.317 L.

Wedge and odd-shaped boxes

Trunk and under-seat boxes often have a sloped back. Split the shape into simple solids: a wedge is a rectangular box plus a triangular prism (half the rectangular volume of its slope section). Always work with internal dimensions, and remember that angled panels have a longer cut edge than the internal dimension they create.

Next steps

Once you know the net volume your driver needs, you can work out the outside dimensions that fit your space. The BassDesign calculator does all of this in one place: use it to work out the net volume, size the port and predict the response before you cut any wood. It is free, needs no sign-up and runs in your browser. Not sure whether to go sealed or ported first? Read sealed vs ported subwoofer box.