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How To Calculate Port Size For Subwoofers

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A properly built subwoofer enclosure with a well-sized port can really enhance bass quality. Sizing the diameter, length and area of the port requires simple math formulas that we will show you. Read on as we explain how to calculate the port size for subwoofers.

So, do you calculate port area?

A subwoofer port size or area is calculated in inches. It is calculated by multiplying the height by the width. For example, if a port measures 4 inches in height by 4 inches in width, then the size or area will be 4 x 4 = 16 sq in. 

Formulas To Calculate Your Enclosure Port Size

The Formula for Port Area
Port area in square inches = height in inches x width in inches
For example, (5 in height and 7 width) – 5 in x 7 in = 35 sq in² of the port area.

The Formula for Port Width
Port width in inches = Port area / height
For example, in our case above, the port area is 35 sq in² and the height is 5 inches, so the width = 35 / 5 = 7 inches

Let’s take another example, let’s say your port area is 64.5 sq in² and the height is 8 inches. In this case, the width = 64.5 / 8 = 8116

To measure port length, measure down the center of the port passage, and to measure port width measure across the passage. 

What Is a Slot Port and How to Make a Slot Port

What Is A Slot Port?

A slot port uses one side of your enclosure walls as a port wall. In other words, it uses the wall of the box and an additional piece of board as the port. For example, if the port is located at one end of the box, you will only need one additional piece of board or wood to complete the port. This means one of the port walls will also be the wall of the box.

subwoofer slot port

In addition to this, the size of the internal wall of the port (between the enclosure wall and the port wall) should be the same as the distance between the end of the port and the back wall of the enclosure. 

In addition, the internal wall should have a space equal to the width of the port between its end and the back wall of the box.

How to Make a Slot Port?

Building a slot port is easy, and is the simplest and easiest way to make a port. Remember, a slot port uses 3 sides of your enclosure as port walls. 

Let’s say you want to place your slot port at the end of the box. In this case, only one additional piece of board or wood will be required to complete the port.

In addition, you need to ensure that the width of the port is the same distance as the distance between the port and the back of the enclosure. 

How To Calculate Port Area?

Subwoofer Port Area Calculation

To calculate port area, multiply the height by the width in inches. The resulting area will be in square inches. For example, a port 4 inches high by 4 inches wide would calculate as 4 x 4= 16 in².

Subwoofer Port Width Calculation

The subwoofer port width is calculated by the following formula:

Port width in inches = Port area / port height
For example, if our port area is 64.5 sq in² and the height is 8 inches, then width = 64.5 / 8 = 8116

In this case, we will round the figure to the nearest ¼ inch and so the measurement will be 81⁄4

In this case, your subwoofer port width should be 81⁄4 inches wide. don’t mind the offset too much. You do have a margin of error to work with. Once you stay within 5-10% of calculated volumes, then any differences will result in very little audible effect.

Port Length Limitations

How To Make a Deep L-Shaped 16-Inch Port For A Small Box

Many people like to work with 16-inch port depths. However, this is not always possible because some box dimensions will simply not allow it because the box may simply be too small.

In this case, you have the choice of bending the port along the back wall to get the full 16″ port length. To calculate where to make the bend, you need to subtract the port width from the inside box depth. For example, if our box depth is 13″ and our port width is 4.75″ then we calculate as follows:

Bend position = box depth – port with = 13″ – 4.75″ = 8.25″

In this case, you would bend the port at 8.25″

Once you bend the port, you need to continue it parallel to the back wall for an additional 16″ – 8.25″ = 7.75″. You will then have a 16″ deep L-shaped port. 

Can You Replace One Large Port With Two Smaller Ports?

Yes, you can replace one large port with 2 small ports, but they must be of the same depth and the area must be the same. For example, consider a power of a 4″ diameter port that has an area of approx 12 sq in. This means if you are using two 2″ diameter ports with the same length a 4″ diameter port will work Ok. 

What Size Port For A 10-Inch Sub?

Below are the dimensions for a 10″ subwoofer:

Parameter Value
Box External Dimensions
Width × Height × Depth
19.69″ × 12.20″ × 15.39″
Material Thickness 0.71″
Net Internal Volume 1.34 ft3
Tuning Frequency 38 Hz
Port Area 16.27 inch2
Port Inlet
Width × Height
1.51″ × 10.79″
Port Length 17.69″


Port Size For 18 Subwoofers

Below are the dimensions for an 18″ subwoofer:

Box External Dimensions
Width × Height × Depth
36.02″ × 20.31″ × 19.69″
Material Thickness 0.71″
Net Internal Volume 5.83 ft3
Tuning Frequency 36 Hz
Port Area 85.24 inch2
Port Inlet
Width × Height
4.51″ × 18.90″
Port Length 19.14″

How To Calculate Subwoofer Port Length?

Subwoofer port length is calculated using a complex mathematical formula. Before we get to it, please note that you can use our subwoofer port length calculator to calculate port length

Here is the subwoofer port length formula:

L = ((2.35625 * 10⁴ * D² * N) / (V * F²)) – (k * D)


L = the length of a port or a vent;
D = the vent’s diameter;
N = the number of ports;
V = the volume of the box;
F = the tuning frequency; and
k = the end correction factor.

Subwoofer Port Length Calculator

A quick and easy way to do these calculations is to use a subwoofer port length calculator. This calculator is used to calculate subwoofer port length.

However, you will need to know a few more parameters. Please check out our subwoofer port calculator to get this done. Here is the quick process:

  1. First, select whether you are using a round or slotted port. Check out our article on round vs slotted ports.
  2. Enter the number of ports, the internal airspace of your enclosure, and the tuning frequency you wish to use
  3. That’s it, just hit calculate
  4. Since you are using a slotted port, select square under port type and select yes to slotted port
  5. Now, under the square vent section, type in the height and width of the port and calculate.

How Do You Calculate Port Displacement?

To calculate the displaced volume of the port, all you need to do is multiply the cross-sectional area by the internal length of the port.

This will give you the volume needed to calculate the displacement. You can use a calculator or even do it by hand.

Can a Subwoofer Port Be Too Long?

If a subwoofer has a port that’s too long, the pressure inside will decrease and this leads to air leaks.

That means you end up with less bass from your system. This can happen if there is excess space around the vent or an opening in front of it. Such as when building into furniture which leaves gaps between panels etc.

How Does Port Area Affect Sound?

Yes, the ports in a speaker help it work better. They make the sound move easier.

When the woofers move back, they push air out. When the woofers move forward, they suck in air.

When you put a speaker in the back, it can’t bounce the sound to you. You need a wall behind it to make the sound louder.

Does Port Shape Matter?

Each angle or corner will result in extra friction and almost double the port length, resulting in poor sq. Adhesives for repairing tiles

In terms of square port length, diameter is far more important than form since adequate size exists. I wouldn’t go above 0.7 feet for a square.

Simply put, the shorter the port the better it will sound.