# What is my subwoofer box volume?

Finds the net inside volume of a sealed or ported subwoofer box from its outside size, wall thickness and driver displacement, and the port length that tunes a ported box to a frequency.

- Page: https://www.acalculator.org/physics/subwoofer-box-calculator
- JSON spec: https://www.acalculator.org/physics/subwoofer-box-calculator.json
- Version: 04d0000436b3

## Default answer

Example with the default inputs (Box type Ported, Outside width 24 in, Outside height 15 in, Outside depth 14 in, Wall thickness 0.75 in, Driver displacement 0.1 ft³, Tuning frequency (Hz) 35, Port diameter 4 in, Number of ports 1, End correction factor 0.732): The net volume is 2.097 ft³ (59.39 L).

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| type | Box type | A sealed box, or a ported (vented) box tuned by a round port. |
| w | Outside width | The box’s outside width. |
| h | Outside height | The box’s outside height. |
| d | Outside depth | The box’s outside depth, front to back. |
| t | Wall thickness | The thickness of the panels; 3/4 in (19 mm) MDF is common. |
| disp | Driver displacement | The volume the subwoofer itself takes up inside the box, from its spec sheet; 0 if unknown. |
| fb | Tuning frequency (Hz) | The frequency the port tunes the box to, in hertz; 30 to 40 Hz is common for subwoofers. |
| pd | Port diameter | The inside diameter of a round port tube. |
| np | Number of ports | How many identical round ports. |
| k | End correction factor | Added acoustic length per port diameter: 0.732 for one end flush with the baffle and one free end. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| net | Net volume (ft³) | Inside volume minus the driver displacement, in cubic feet. |
| netL | Net volume (liters) | The net volume in liters. |
| gross | Inside volume (ft³) | Inside width × height × depth, before the driver. |
| inside | Inside size | Each outside size minus two wall thicknesses. |
| portLength | Port length (in) | The length of each port tube for the tuning frequency. |
| portCm | Port length (cm) | The port length in centimeters. |
| portVolume | Port volume (ft³) | The volume the port tubes take up; add it to the box if you want this net volume after the ports. |

## Method

Inside size = outside − 2 × wall; net volume V = inside width × height × depth − driver displacement. Port: L_eff = c² × A ÷ (4π² × f² × V), with A the total port area and c = 343 m/s; port length = L_eff − k × D.

## Assumptions

- Rectangular box with all six walls the same thickness; bracing and the port tubes are not subtracted (the port volume is shown so you can add it).
- Speed of sound 343 m/s (air at 20 °C). Round ports; several ports act as one port of their total area.
- The end correction k × D (default 0.732, one flanged and one free end) is the usual design value; real boxes may tune a few hertz off.

## Worked examples

1. type = sealed, w = 0.6096, h = 0.381, d = 0.3556, t = 0.01905, disp = 0.002832 gives gross = 2.197266, net = 2.097266, inside = 22.5 in × 13.5 in × 12.5 in. Source: NIST Special Publication 811 (1 in = 2.54 cm exactly, so 1 ft³ = 1,728 in³), https://www.nist.gov/pml/special-publication-811.
2. type = ported, w = 0.5, h = 0.5, d = 0.5, t = 0, disp = 0, fb = 35, pd = 0.1016, np = 1, k = 0.732 gives netL = 125, portLength = 3.283922. Source: Wikipedia, Helmholtz resonance (f = (v ÷ 2π) × √(A ÷ (V₀ × L_eq)), with L_eq the neck length plus an end correction), https://en.wikipedia.org/wiki/Helmholtz_resonance (retrieved 2026-10-02); OpenStax, University Physics Volume 1, §17.2 Speed of Sound (343 m/s in air at 20 °C), https://openstax.org/books/university-physics-volume-1/pages/17-2-speed-of-sound (retrieved 2026-10-02).
3. type = ported, w = 0.6096, h = 0.3048, d = 0.3048, t = 0, disp = 0, fb = 35, pd = 0.1016, np = 1, k = 0.732 gives net = 2, portLength = 10.782747. Source: Wikipedia, Helmholtz resonance (f = (v ÷ 2π) × √(A ÷ (V₀ × L_eq)), with L_eq the neck length plus an end correction), https://en.wikipedia.org/wiki/Helmholtz_resonance (retrieved 2026-10-02); OpenStax, University Physics Volume 1, §17.2 Speed of Sound (343 m/s in air at 20 °C), https://openstax.org/books/university-physics-volume-1/pages/17-2-speed-of-sound (retrieved 2026-10-02).

## FAQ

### How do I calculate subwoofer box volume?

Take two wall thicknesses off each outside size to get the inside size, multiply the three, and divide by 1,728 to turn cubic inches into cubic feet. A 24 × 15 × 14 in box of 3/4 in MDF is 22.5 × 13.5 × 12.5 = 3,796.9 in³, or 2.197 ft³ inside.

### What is net volume?

The air space the subwoofer actually works with: the inside volume minus what the driver itself takes up (its displacement, from the spec sheet), and minus bracing and ports if you subtract them. Subtracting a 0.1 ft³ driver from 2.197 ft³ leaves 2.097 ft³.

### How long should my port be?

A ported box is a Helmholtz resonator: f = (c ÷ 2π) × √(A ÷ (V × L_eff)). Solving for the length gives L_eff = c² × A ÷ (4π² × f² × V), and the tube is L_eff minus an end correction. A 2 ft³ box tuned to 35 Hz with one 4 in port needs about 10.8 in.

### What tuning frequency should I pick?

Subwoofer boxes are often tuned around 30 to 40 Hz. Lower tuning reaches deeper but needs a longer port or a bigger box. Check your driver maker's advice for the box size and tuning that suit it.

### What is the end correction?

Air just outside each end of the port moves with the air inside, so the port acts longer than the tube. The usual design value is 0.732 × the port diameter, for one end flush with the baffle and one free end inside the box. You can change it under More options.

### Does the port take up box volume?

Yes. The page shows the port volume, π × (D ÷ 2)² × L for each port, so you can add it to the box size if you want the net volume left after the ports.

## Sources

- Wikipedia, Helmholtz resonance (resonant frequency f = (v ÷ 2π) × √(A ÷ (V₀ × L_eq)), with L_eq the neck length plus an end correction). https://en.wikipedia.org/wiki/Helmholtz_resonance (retrieved 2026-10-02)
- OpenStax, University Physics Volume 1, §17.2 Speed of Sound (the speed of sound in air is 343 m/s at 20 °C). https://openstax.org/books/university-physics-volume-1/pages/17-2-speed-of-sound (retrieved 2026-10-02)
- Wikipedia, End correction (the added length at an open pipe end; about 0.6133 × the radius for an unflanged end, Levine and Schwinger 1948). https://en.wikipedia.org/wiki/End_correction (retrieved 2026-10-02)
- NIST Special Publication 811, Guide for the Use of the International System of Units, Appendix B (1 in = 2.54 cm exactly). https://www.nist.gov/pml/special-publication-811 (retrieved 2026-10-02)