acalculator

What is my subwoofer box volume?

Type the outside width, height and depth of your box, the wall thickness and the driver's displacement. The subwoofer box calculator gives the net inside volume in cubic feet and liters. For a ported box, it also gives the length of round port that tunes the box to the frequency you choose.

Your numbers

Units
Box type
More options
Port length (in)
10.15

The net volume is 2.097 ft³ (59.39 L).

Net volume (ft³)
2.097
Net volume (liters)
59.39
Inside volume (ft³)
2.197
Inside size
22.5 in × 13.5 in × 12.5 in
Port length (cm)
25.8
Port volume (ft³)
0.0738

Port length (in): 10.15. The net volume is 2.097 ft³ (59.39 L).

How to calculate

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.

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).

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.

  • 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.

Machine-readable copies: Markdown, JSON.

Worked examples

Each example is checked against the calculator on every build.

  1. Box type Sealed, Outside width 24 in, Outside height 15 in, Outside depth 14 in, Wall thickness 0.75 in, Driver displacement 0.1 ft³ gives Inside volume (ft³) 2.197266, Net volume (ft³) 2.097266, Inside size 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. Box type Ported, Outside width 19.69 in, Outside height 19.69 in, Outside depth 19.69 in, Wall thickness 0 in, Driver displacement 0 ft³, Tuning frequency (Hz) 35, Port diameter 4 in, Number of ports 1, End correction factor 0.732 gives Net volume (liters) 125, Port length (in) 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. Box type Ported, Outside width 24 in, Outside height 12 in, Outside depth 12 in, Wall thickness 0 in, Driver displacement 0 ft³, Tuning frequency (Hz) 35, Port diameter 4 in, Number of ports 1, End correction factor 0.732 gives Net volume (ft³) 2, Port length (in) 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)

How it works

Box volume. For outside width W, height H and depth D and wall thickness t:

  • inside size = (W − 2t) × (H − 2t) × (D − 2t)
  • net volume V = inside volume − driver displacement
  • 1 ft³ = 0.3048³ m³ = 1,728 in³; 1 L = 0.001 m³

Port length (ported box). The box and port act as a Helmholtz resonator. With n round ports of diameter d, total area A = n × π × (d ÷ 2)², speed of sound c = 343 m/s and tuning frequency f:

  • f = (c ÷ 2π) × √(A ÷ (V × L_eff)), so L_eff = c² × A ÷ (4π² × f² × V)
  • port length = L_eff − k × d, where k is the end correction factor (default 0.732)
  • port volume = A × port length

Lengths are converted to meters (1 in = 0.0254 m), volumes to cubic meters, and the maths runs in double precision; answers are turned back into feet, inches, liters and centimeters.

Rules

  • Outside sizes and the port diameter 1 mm to 10 m; wall thickness 0 to 10 m; displacement 0 to 10 m³. Each outside size must be more than two wall thicknesses, and the displacement less than the inside volume (a net volume within 10⁻¹² of the inside volume of 0 has no answer).
  • Tuning 5 Hz to 200 Hz; 1 to 4 ports; end correction factor 0 to 1.
  • If L_eff is no more than k × d, no port length works (the end correction alone tunes lower), and there is no answer.
  • Bracing, port volume and the air in the driver's cone are not subtracted. The model ignores port air speed (chuffing) and box losses.

Output format. Volumes to 4 significant figures (port volume to 3); port length to 2 decimal places in inches and 1 in centimeters. The inside size reads "22.5 in × 13.5 in × 12.5 in", each rounded to 2 decimals.

Worked examples by hand

Sealed, 24 × 15 × 14 in, 3/4 in walls, 0.1 ft³ driver. Inside 22.5 × 13.5 × 12.5 = 3,796.875 in³ = 2.197 ft³; net 2.197 − 0.1 = 2.097 ft³.

Ported, 125 L, one 4 in port, 35 Hz. A = π × 0.0508² = 0.0081073 m²; L_eff = 343² × 0.0081073 ÷ (4π² × 35² × 0.125) = 0.15778 m; port = 0.15778 − 0.732 × 0.1016 = 0.08341 m = 3.28 in.

Ported, 2 ft³, one 4 in port, 35 Hz. V = 0.056634 m³; L_eff = 0.34825 m = 13.71 in; port = 13.71 − 0.732 × 4 = 10.78 in.

Other questions people ask

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.