What end-correction factor should I use?

It depends on how the port mounts. A straight tube hanging in free air uses the both-free factor (1.227). A port whose front is flush with the baffle uses one-flanged (1.463). A port flanged at both ends uses 1.728. The factor multiplies the port radius and is subtracted from the ideal length.

Why does port diameter change the length so much?

Port length scales with port area, and area grows with the square of the diameter. Doubling the diameter roughly quadruples the area, which lengthens the tube substantially for the same tuning. That is why small-diameter ports are short but prone to chuffing, while large ports are long but quiet.

What if the calculator returns a negative length?

A negative or zero result means the port area is too large for that box and tuning — the end correction alone exceeds the ideal length. Use a smaller diameter, fewer ports, or a higher tuning frequency so a physical tube length exists.

Does multiple ports change the length each one needs?

Yes. The formula uses total port area, so adding ports increases area and shortens each individual tube. Each of two identical ports is shorter than one single port of the same diameter, while together they move more air at lower velocity.

How accurate is this versus a full box simulator?

The Helmholtz length formula is accurate to within a few percent for typical round ports. Real-world factors — port flares, proximity to walls, and box losses — shift tuning slightly, so cut the port a little long and verify the final tuning by measurement, trimming as needed.

Bass Reflex Port Length Calculator

The port (or vent) in a bass-reflex speaker enclosure is a tube of air whose mass resonates with the box volume. Get the length wrong and the box tunes to the wrong frequency. This calculator returns the precise tube length to cut for any box volume, port diameter and target tuning frequency.

How it works

The tuning frequency of a ported box is governed by the Helmholtz resonator equation. Rearranged to solve for the port tube length:

Lv = ( c² · Av ) / ( 4 · π² · Fb² · Vb ) − k · √(Av / π)

Here c is the speed of sound (34 400 cm/s), Av is total port area in cm², Vb is the net box volume in cm³, Fb is the tuning frequency, and k is the end-correction constant. The total port area is π · (d/2)² per tube times the number of ports.

The subtracted term, k · √(Av / π), is the acoustic end correction: air just outside each opening oscillates along with the column inside, so the tube behaves acoustically longer than its physical length. Subtracting it gives the real length you cut.

Tips and example

For a 40 L box tuned to 32 Hz with a single 7.5 cm flush-mounted port, the area is about 44.2 cm² and the calculated length is roughly 27 cm. Switching to a both-flanged port (factor 1.728) shortens that result slightly because more end correction is removed.

Match the end-correction setting to how the port physically mounts.
Larger or additional ports lower air velocity but lengthen the tube.
Cut slightly long, measure the actual tuning, then trim — tuning rises as you shorten.

All maths runs in your browser; no data leaves your device.