Ported Box Calculator

✅ Reviewed by Markus Vance, AES Member | Audio Acoustics Engineering Specialist

Optimize your subwoofer enclosure performance. Use this Ported Box Calculator to determine the ideal port length, box volume, or tuning frequency for your DIY audio build.

Leave one field blank to solve for it.

Net Box Volume ($V_b$) (ft³)

Tuning Frequency ($F_b$) (Hz)

Port Diameter ($D$) (inches)

Standard round port inner diameter

Port Length ($L_v$) (inches)

Enter 3 variables and click Calculate

Ported Box Calculator Formula

$L_v = \frac{1.463 \times 10^7 \times R^2}{F_b^2 \times V_b} – 1.463R$ Source: JL Audio Enclosure Technical Support

Variables Explained:

$V_b$ (Net Box Volume): The internal volume of the enclosure after subtracting the displacement of the driver and port.
$F_b$ (Tuning Frequency): The resonant frequency of the port/enclosure system, measured in Hertz (Hz).
$D$ (Port Diameter): The inside diameter of the vent tube. $R$ is the radius ($D/2$).
$L_v$ (Port Length): The physical length of the port required to achieve the target tuning frequency.

Related Calculators

What is a Ported Box?

A ported box, also known as a bass reflex or vented enclosure, uses a “port” (usually a hollow tube or slot) to reinforce the bass response of a subwoofer. Unlike sealed boxes, ported enclosures allow air to move in and out, which uses the backwave of the speaker to increase output at specific frequencies.

The efficiency of a ported box is significantly higher than a sealed box, making it the preferred choice for those looking for high output and “boomier” bass. However, accurate calculation is critical; an improperly tuned port can cause mechanical failure of the subwoofer due to over-excursion.

How to Calculate Ported Box Tuning (Example)

Determine your target **Tuning Frequency** (e.g., 32 Hz for deep bass).
Measure the **Net Volume** of your box (e.g., 2.0 cubic feet).
Choose a **Port Diameter** that minimizes air noise (e.g., 4 inches).
Apply the formula to find the **Port Length**. For the above, the length would be approximately 19.5 inches.

Frequently Asked Questions (FAQ)

What happens if my port is too long?

If the port is too long for the box size, the tuning frequency ($F_b$) will drop, potentially resulting in a “mushy” bass response and higher risk of driver damage below tuning.

Is a round port better than a slot port?

Functionally, they work the same way. Slot ports are often used because they can also act as internal bracing, while round ports are easier to adjust during the building process.

Why does the box volume matter for tuning?

The air inside the box acts as a spring. A larger volume of air is “softer,” requiring a longer port to achieve the same resonance as a smaller, “stiffer” box volume.

What is EBP?

Efficiency Bandwidth Product (EBP) is a ratio used to determine if a speaker is best suited for a ported (EBP > 50-100) or sealed (EBP < 50) enclosure.