Zerdly

Subwoofer Port Length Calculator

Calculate the length of a round vent (port) needed to tune a ported bass-reflex enclosure to a target frequency. The calculation uses the Helmholtz resonator relationship between box volume, port area and tuning frequency.

100% FreeRuns in your browserIntermediate
How to use this calculator

Enter Values

Enter your values and press Calculate to see results.

How to use this calculator

  1. Enter the net internal volume of the box in litres.
  2. Enter the tuning frequency you want (Fb) in hertz and the internal diameter of the port in centimetres.
  3. Set the number of ports if you are using more than one, then read the required length for each port.

How it works

Port length Lv (cm) = (23562.5 × d² × Np) ÷ (Fb² × Vb) − 0.732 × d, where d is the port internal diameter in cm, Np the number of ports, Fb the tuning frequency in Hz and Vb the net box volume in litres. The −0.732 × d term is the end correction for a port flush on one side.

Worked example

Worked example. For a 40 L box tuned to 35 Hz with one 7.5 cm port: Lv = (23562.5 × 7.5² × 1) ÷ (35² × 40) − 0.732 × 7.5 = 27.05 − 5.49 = 21.56 cm (about 8.49 in).

Common mistakes

  • Using the gross box volume instead of the net internal volume for Vb.
  • Choosing a port diameter so small that the vent length becomes impractically long — increase the diameter or add ports.
  • Ignoring port air velocity, which causes audible 'chuffing' when a single small port moves too much air.

Frequently asked questions

My port length is huge — what should I do?

A long port usually means the diameter is too small for the box and tuning. Fit a larger-diameter port or use two ports; both increase the vent area and shorten the required length.

Does this work for slot (rectangular) ports?

This calculator assumes round ports. For a slot port, first work out the equivalent circular diameter that gives the same cross-sectional area, then use that value here as an approximation.

Related tools

Explore more in Audio, Acoustics & AV.

Tip: Enter any known values to calculate the remaining results.

All calculations run in your browser. Your inputs are never saved or transmitted.