Port Size Calculator
Enter the box's net air volume, the frequency you want to tune it to, the port's inside diameter and how many ports there are. The calculator gives the port length, the total port area and whether it fits in the box.
- Port length from tuning
- Round ports, one or more
- Fit check
Use the net volume: the box's inside volume less the driver, bracing and the port itself.
A port turns a sealed speaker box into a tuned one. The air in the port bounces on the air in the box like a spring, boosting bass near the tuning frequency. A smaller box or lower tuning needs a longer port for the same diameter.
Port length chart
4 in port, 2 ft³ box. Search for a tuning.
| Tuning (Hz) | Port length (in) | Port length (cm) |
|---|---|---|
| 25 | 24 | 61 |
| 30 | 15.7 | 40 |
| 35 | 10.8 | 27 |
| 40 | 7.6 | 19 |
| 45 | 5.4 | 14 |
| 50 | 3.8 | 10 |
| 60 | 1.7 | 4 |
Longer ports than the box depth need an elbow or a slot port along a wall.
Tuning and port length
2 ft³ box, 4 in port. Slide the tuning.
The port is drawn to scale inside the box. A very long port bends up along the side wall.
How to size a port
Box, tuning, diameter. Step through it.
Net volume
Inside air space.
The formula is L = 2,100 × D² × N ÷ (Fb² × Vb) − 0.732 × D, in inches and cubic feet.
Round, slot or passive radiator
Port types. Compare them.
A tube.
A wide channel along a wall.
A cone with no magnet.
Port noise starts when air speed passes about 17 m/s. A larger port area keeps it quiet but needs more length.
Before cutting
Box checks. Tick each item.
Tick each item as you go.
Make the port a little long, measure the tuning, then trim to raise it.
What you get
Port design. Each tile explains one result.
Inches or cm.
All ports.
ft³ and litres.
Hz.
In the box.
A flared port end reduces noise and lets you use a slightly smaller port.
Frequently Asked Questions
No question matches that search.
How do I calculate port length for a speaker box?
L = 2,100 × D² × N ÷ (Fb² × Vb) − 0.732 × D, with D in inches and Vb in cubic feet.
What frequency should I tune my sub box to?
About 30 to 35 Hz for music, a little lower for home cinema.
Does a longer port lower the tuning?
Yes. A longer port tunes the box lower.
What happens if the port is too small?
It makes a chuffing noise at high volume as air rushes through.
Can I use two ports instead of one?
Yes. Two ports need to be longer than one of the same diameter.
What if the port is longer than the box?
Use an elbow, or a slot port that runs along a wall.
Does port volume count as box volume?
No. Subtract the port's own volume from the box to get the net volume.
Bass tuned to taste
Choose the tuning, the port size and the box volume, and get the length to cut. Then fine tune by trimming the port.
More size calculators
Each one has its own calculator, worked examples and answers to common questions.