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Port Length Calculator

Table of contents

How to use the subwoofer port length calculator?How to calculate subwoofer port length?What is the end correction factor (k)?Why get a subwoofer in the first place?

If you need to quickly calculate the length of the vent for your subwoofer, you're in the right place - this subwoofer port length calculator 🔊 was created precisely for that purpose!

Use it to estimate the appropriate length of the vent in your subwoofer, and read on to find out the math behind the process and learn the subwoofer port length formula.

💡 Since you're interested in the port length for a subwoofer, perhaps you're DIYing a speaker. If you haven't made your speaker enclosure yet, you might find our speaker box calculator helpful. Our crossover calculator can help you with the electrical circuitry of your speaker.

How to use the subwoofer port length calculator?

To have your subwoofer port's length estimated, you need to provide the calculator with some basic information about the subwoofer box. Please follow these instructions, and you'll be done in no time!

  • Start by inputting the number of ports (N\small N) on the subwoofer box you're building.
  • Follow by inputting the diameter (D\small D) of the vent(s).
  • Then, fill in the volume (V\small V) of the subwoofer box.
  • Input the tuning frequency (F\small F). Learn about the frequency of sound waves by checking out our frequency calculator.
  • Finally, adjust the end correction factor (k\small k). It is common to assume that k=0.732\small k=0.732, hence the default value set in the port length calculator. Feel free to change it to fit your needs.

That's all! When all the fields are filled with numbers, the port length calculator combines them and returns the subwoofer port length you need. You will see the result in the bottom-most field of the tool.

As with any Omni Calculator, you can do the calculation any way you want. Just input the fields you know.

How to calculate subwoofer port length?

You're probably interested in the math behind the workings of this subwoofer port calculator. It might seem complicated when you look at the many different variables you have to fill in, but don't worry! What it comes down to is one relatively simple subwoofer port length formula:

L=2.35625×104×D2×NV×F2(k×D),\footnotesize L = \frac{2.35625 \times 10^4 \times D^2\times N}{V\times F^2} - (k\times D),


  • LL - Length of a port or a vent;
  • DD - Vent's diameter;
  • NN - Number of ports;
  • VV - Volume of the box;
  • FF - Tuning frequency; and
  • kk - End correction factor.

What is the end correction factor (k)?

In short, end correction is the short distance that you add to the end of a resonance pipe in order to figure out its resonant frequency. In other words, it is a measurement of standing waves. It mostly depends on the radius of the tube. Knowing its value is necessary for accurate calculations of port length.

In subwoofer box calculations, we assumed an end correction factor of 0.732. The value might change depending on the shape of the pipe used as the vent in the following way:

  • Both ends flanged: k=0.850\small k = 0.850;
  • One end flanged, one free (unflanged): k=0.732\small k = 0.732; and
  • Both ends free: k=0.614\small k = 0.614.
Port length calculator: pipe ends.

Why get a subwoofer in the first place?

Since you're using this calculator, you probably don't need any more convincing that subwoofers are worth it. But, on the off chance you're only browsing put of curiosity, here's a short subwoofer 101 description!

What's a subwoofer? Essentially, it's a type of speaker. The reason it stands out from other speakers is that it is capable of delivering low frequencies (20 - 200 Hz). Importantly, traditional sounds systems can't produce them.

Why is it cool? If you want to be able to enjoy listening to such instruments as bass guitar and pipe organ to the full extent, or even feel the explosions while watching movies rather than just hear them, a subwoofer is the way to go. Subwoofers allow for much more immersive listening experiences than traditional sound systems. If you enjoy music, why you should get it is self-explanatory. However, it can also turn watching movies and playing video games into a whole different level of experience too! Especially in the former case, installing a subwoofer in your home theater system is a great idea, as it will take you much closer to the feel of watching a movie in an actual cinema.

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