Subwoofer Box Volume Calculator (Sealed)

Enter your driver's Vas and Qts, plus your target system Qtc, to calculate the ideal sealed box volume in litres and cubic feet.

Created by Luis GonzalezLast updated: May 9, 2026

How to Use This Calculator

1
Enter Driver Vas (L)
Input the equivalent compliance volume of your subwoofer driver in litres. This is a key Thiele-Small parameter found in manufacturer specs.
2
Provide Driver Qts
Enter the total Q factor of the driver (Qts), which indicates its damping characteristics. Typical values range from 0.2 to 0.9.
3
Set Target System Qtc
Input your desired system Q factor (Qtc) for the sealed enclosure. 0.707 is for a maximally flat (Butterworth) response; lower values yield tighter bass, higher values add bass lift.
4
Review your results
The calculator will display the ideal sealed box volume in litres and cubic feet, along with alignment type and stuffing equivalent.

Example Calculation

An audio enthusiast wants to design a sealed enclosure for a subwoofer driver with Vas of 50 L and Qts of 0.4, aiming for a maximally flat (Butterworth) Qtc of 0.707.

Driver Vas (L)

Driver Qts

0.4

Target System Qtc

0.707

Results

23.53 L

Tips

Match Qtc to Your Listening Preferences

A Qtc of 0.707 provides a maximally flat frequency response, often preferred for accuracy. Lower Qtc values (e.g., 0.5-0.6) offer tighter, more articulate bass, while higher values (e.g., 0.8-0.9) provide more mid-bass 'punch' but can sound boomy.

Consider Acoustic Stuffing for Smaller Boxes

Adding acoustic stuffing (e.g., polyfill, fiberglass) to your sealed enclosure can effectively increase its internal volume by 10-20% without physically enlarging the box. This is useful for achieving a desired Qtc with a more compact design.

Factor in Driver Displacement and Bracing

The calculated box volume is the *net* internal air volume. Remember to subtract the volume displaced by the subwoofer driver itself and any internal bracing to ensure your physical box matches the theoretical volume.

Designing Optimal Sealed Subwoofer Enclosures

The Subwoofer Box Volume Calculator (Sealed) is an essential tool for audio enthusiasts and engineers, precisely determining the ideal enclosure volume from key driver parameters like Vas and Qts, and a target Qtc. It provides critical insights into alignment type, stuffing equivalent, and the Vb/Vas ratio. Achieving the correct sealed box volume is paramount for maximizing bass quality, ensuring a driver operates efficiently, and delivering the desired frequency response for any audio system in 2025.

Engineering Optimal Bass Performance with Sealed Enclosures

Precise box volume is the bedrock of a high-performing sealed subwoofer. A correctly designed enclosure ensures the driver operates within its optimal parameters, preventing unwanted resonances and achieving a desired frequency response. For instance, aiming for a system Q factor (Qtc) of 0.707 results in a maximally flat (Butterworth) response, providing accurate, tight bass without boominess. Deviations from this ideal can lead to "one-note" bass or a lack of definition. Understanding how parameters like Vas (the driver's equivalent air volume) and Qts (its total damping) interact with the enclosure size is crucial for engineering a subwoofer that integrates seamlessly into any audio setup.

The Thiele-Small Formula for Sealed Box Volume

The calculation for the ideal sealed subwoofer box volume (Vb) is derived from the driver's Thiele-Small parameters, specifically Vas and Qts, and your desired system Q factor (Qtc).

ratio = (Qtc^2 / Qts^2) - 1
box volume (Vb) = Vas / ratio

The Driver Vas (equivalent compliance volume) is typically measured in litres, and the Driver Qts and Target System Qtc are dimensionless ratios. The result, Box Volume, is given in litres, cubic feet, and cubic inches, providing flexibility for construction. The calculator also estimates the stuffing equivalent, which can effectively increase the volume by 15-20% with acoustic fill.

💡 To fine-tune your audio system, our Speaker Sensitivity Calculator can help you understand how much power your speakers need to reach desired volume levels.

Calculating a Sealed Box for a Custom Subwoofer

Let's design a sealed enclosure for a new subwoofer driver using the default specifications:

Enter Driver Vas (L): 50 litres.
Provide Driver Qts: 0.4.
Set Target System Qtc: 0.707 (for a maximally flat response).

The calculator first computes the ratio: (0.707^2 / 0.4^2) - 1 = (0.500 / 0.16) - 1 = 3.125 - 1 = 2.125. Then, Box Volume (Vb) = 50 L / 2.125 = 23.53 litres. This volume translates to approximately 0.83 cubic feet or 1436 cubic inches. The Qtc Alignment is confirmed as "Butterworth (maximally flat)," indicating a balanced and accurate bass response for the system.

💡 When setting up your entire audio system, our Speaker Wire Resistance Loss Calculator ensures optimal power delivery from your amplifier to your speakers.

Comparing Sealed vs. Ported Subwoofer Enclosures

While sealed (acoustic suspension) enclosures are known for their tight, accurate, and controlled bass response, especially for musical reproduction, they are not the only option. Ported (bass reflex) enclosures, which feature a tuned vent or port, offer a different set of acoustic characteristics. Ported designs typically achieve higher output levels and can extend deeper into the lower frequencies than a sealed box of the same size, often at the cost of transient response and a less linear roll-off. The choice between sealed and ported depends on the desired sound profile: sealed for accuracy and definition, ported for maximum impact and low-frequency extension. Each type requires different calculation methods and Thiele-Small parameters for optimal design.

Frequently Asked Questions

What are Thiele-Small parameters and why are they important for subwoofer design?

Thiele-Small parameters are a set of electromechanical specifications that define a loudspeaker driver's low-frequency performance in a given enclosure. Key parameters like Vas (equivalent compliance volume), Qts (total Q factor), and Fs (resonant frequency) are critical for accurately calculating the ideal box volume and predicting the frequency response of a subwoofer system.

What does a Qtc of 0.707 mean for a sealed subwoofer enclosure?

A Qtc (system Q factor) of 0.707 for a sealed subwoofer enclosure represents a Butterworth alignment, which delivers the flattest possible frequency response with the best transient response. This is often considered the ideal target for accurate, musical bass reproduction, as it provides a smooth roll-off without excessive peaks or dips.

How does acoustic stuffing affect the effective volume of a sealed subwoofer box?

Acoustic stuffing, such as polyfill or fiberglass, affects the effective volume of a sealed subwoofer box by slowing down the speed of sound within the enclosure. This makes the driver 'think' the box is larger than its physical dimensions, typically increasing the effective volume by 10-20%. It helps achieve a desired Qtc in a smaller physical footprint.