How do I calculate the volume of a wedge subwoofer box?

Average the two depths. A truck wedge box with a top depth of 4 inches and a bottom depth of 10 inches has an average depth of 7 inches, so a 30 inch wide, 14 inch tall wedge is 30 x 14 x 7 = 2,940 cubic inches, or 1.70 cubic feet, before subtracting the wood, the driver displacement and the port. Enter that net figure here to check tuning.

Is this the same as the Rockford Fosgate box wizard?

It uses the same Thiele-Small math: box volume, tuning frequency and port length from the driver's Vas, Qts and Fs. The difference is that this calculator shows the formula and works for any driver, while the Rockford Fosgate wizard is built around that brand's own subwoofers and recommended enclosures. If you own a Rockford sub, run both; they should agree within a few percent.

What port area and port size do I need?

The common convention is 12 to 16 square inches of port area per cubic foot of net box volume for a daily system, rising toward 20 for high-power SPL builds to avoid port noise. A 2 cubic foot box therefore wants a port of roughly 24 to 32 square inches, for example 4 x 7 inches, and the port length then follows from the tuning frequency, which this calculator returns.

How do I calculate subwoofer box volume in cubic feet?

Multiply the internal width, height and depth in inches, then divide by 1,728, the number of cubic inches in a cubic foot. A box measuring 28 by 14 by 13 inches inside is 5,096 cubic inches, or about 2.95 cubic feet gross. Then subtract the space taken by the port, bracing and the subwoofer to get net volume, which is the number to match against the recommended box size.

Are these figures published or are they rules of thumb?

Both, and the page labels which is which. The Helmholtz port relation, the Thiele/Small alignment equations, cone area geometry, Ohm’s law, conductor resistance and ampacity, and manufacturer specifications are published and are labeled as such. Amplifier efficiency, cabin gain, the farad per thousand watts rule, port area per cubic foot and coverage percentages are conventions with no measurement behind them, and they are labeled as conventions.

Why does this calculator sometimes refuse to give an answer?

Because there genuinely is not one. A port can be too large in area for the box and tuning asked of it, a vented alignment stops describing a useful box outside a Qts range, and a sealed box cannot produce a total Q below the driver’s own. In each case the calculator returns nothing and explains why, rather than extrapolating past the end of a published relation.

Do I still need an installer if I use this?

Many owners do their own 12V work, and these are the figures that work is sized from. What matters is that a car battery delivers hundreds of amps into a short without hesitating, so the main fuse goes within 18 inches of the battery positive terminal and the battery comes off before power wire goes in. If any of that is beyond what you want to take on, a professional installer is money well spent.

Enclosure calculator

Sub Box Calculator: Ported Volume (Cubic Feet), Port and Tuning

Updated 2026-09-26. Researched from published standards and manufacturer specifications.

Quick answer

The QB3 vented alignment gives Vb = 20 x Vas x Qts^3.3, tuned to Fb = 0.42 x Fs x Qts^-0.96. A driver with Vas of 1.2 cubic feet, Qts of 0.42 and Fs of 32 Hz wants about 1.7 cubic feet tuned near 31 Hz.

Converting internal box dimensions to cubic feet (internal inches multiplied, divided by 1,728)
Internal dimensions (W x H x D)	Gross volume	Typical use
24 x 12 x 12 in	2.00 cu ft	Single 10 inch, ported
28 x 14 x 13 in	2.95 cu ft	Single 12 inch, ported
32 x 15 x 15 in	4.17 cu ft	Single 15 inch or two 10s
36 x 15 x 16 in	5.00 cu ft	Two 12 inch, ported

Published figure Source: Volume is length x width x height; 1 cubic foot is 1,728 cubic inches. The typical-use column is a common convention, not a specification. Measure inside the box, not outside. Subtract the volume the port, bracing and the subwoofer itself displace to get net volume, then compare net volume to the calculator result and the manufacturer range.

Result

Net internal volume 1.371 cu ft Published figure

Tuning frequency Fb 30.9 Hz Published figure

Predicted F3 28 Hz Published figure

In cubic inches 2369 cu in

This is NET volume after the port and the driver are subtracted. Work out the port next, then add its displacement back into the box you build.

The formula

Vb = 20 x Vas x Qts^3.3, Fb = 0.42 x Fs x Qts^-0.96, F3 = 0.26 x Fs x Qts^-1.4

QB3 only describes a useful vented box over roughly Qts 0.2 to 0.5. Outside that range this returns nothing rather than a number that looks authoritative and is not. A driver with a Qts above 0.5 generally wants a sealed box.

The figures above are computed with these exact inputs: Vas 1.2 cu ft, Qts 0.42, Fs 32 Hz.

What this calculator assumes

Some of these are published physics or manufacturer figures and some are conventions. They are not the same kind of number, so each one is labeled.

Assumption	Kind	Where it comes from
Thiele and Small vented-box alignment approximations	Published figure	Thiele and Small vented-box alignment approximations, quasi-Butterworth third order (QB3)

An armor coated box with a kerf port, which survives a truck bed and a move far better than carpet over MDF.

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The Car Audio Build & Tuning Planner

That vented volume is the figure the Speaker & Enclosure Sizing Sheet logs alongside the tuning frequency and the port length that goes with it.

10 fill-in worksheets, 29 print pages
2 filled-in examples to copy the format from
Companion Excel / Google Sheets tracker with the wire gauge and box volume formulas built in
Online buyer guide with budget ranges and Amazon links, by build tier

Replaces the guessed wire gauge, the fuse sized to the amplifier instead of the wire, the gain set by ear, and the ground point picked without checking for bare metal.

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Frequently asked questions

How do I calculate the volume of a wedge subwoofer box?: Average the two depths. A truck wedge box with a top depth of 4 inches and a bottom depth of 10 inches has an average depth of 7 inches, so a 30 inch wide, 14 inch tall wedge is 30 x 14 x 7 = 2,940 cubic inches, or 1.70 cubic feet, before subtracting the wood, the driver displacement and the port. Enter that net figure here to check tuning.
Is this the same as the Rockford Fosgate box wizard?: It uses the same Thiele-Small math: box volume, tuning frequency and port length from the driver's Vas, Qts and Fs. The difference is that this calculator shows the formula and works for any driver, while the Rockford Fosgate wizard is built around that brand's own subwoofers and recommended enclosures. If you own a Rockford sub, run both; they should agree within a few percent.
What port area and port size do I need?: The common convention is 12 to 16 square inches of port area per cubic foot of net box volume for a daily system, rising toward 20 for high-power SPL builds to avoid port noise. A 2 cubic foot box therefore wants a port of roughly 24 to 32 square inches, for example 4 x 7 inches, and the port length then follows from the tuning frequency, which this calculator returns.
How do I calculate subwoofer box volume in cubic feet?: Multiply the internal width, height and depth in inches, then divide by 1,728, the number of cubic inches in a cubic foot. A box measuring 28 by 14 by 13 inches inside is 5,096 cubic inches, or about 2.95 cubic feet gross. Then subtract the space taken by the port, bracing and the subwoofer to get net volume, which is the number to match against the recommended box size.
How does the ported box volume calculation work?: Vb = 20 x Vas x Qts^3.3, Fb = 0.42 x Fs x Qts^-0.96, F3 = 0.26 x Fs x Qts^-1.4. QB3 only describes a useful vented box over roughly Qts 0.2 to 0.5. Outside that range this returns nothing rather than a number that looks authoritative and is not. A driver with a Qts above 0.5 generally wants a sealed box.
Are these figures published or are they rules of thumb?: Both, and the page labels which is which. The Helmholtz port relation, the Thiele/Small alignment equations, cone area geometry, Ohm’s law, conductor resistance and ampacity, and manufacturer specifications are published and are labeled as such. Amplifier efficiency, cabin gain, the farad per thousand watts rule, port area per cubic foot and coverage percentages are conventions with no measurement behind them, and they are labeled as conventions.
Why does this calculator sometimes refuse to give an answer?: Because there genuinely is not one. A port can be too large in area for the box and tuning asked of it, a vented alignment stops describing a useful box outside a Qts range, and a sealed box cannot produce a total Q below the driver’s own. In each case the calculator returns nothing and explains why, rather than extrapolating past the end of a published relation.
Do I still need an installer if I use this?: Many owners do their own 12V work, and these are the figures that work is sized from. What matters is that a car battery delivers hundreds of amps into a short without hesitating, so the main fuse goes within 18 inches of the battery positive terminal and the battery comes off before power wire goes in. If any of that is beyond what you want to take on, a professional installer is money well spent.

Researched guidance, not professional advice. These results are planning figures from published relations and manufacturer specifications. Fit the main fuse within 18 inches of the battery positive terminal, because that fuse protects the wire and the wire is what starts a fire. Disconnect the battery before running power wire, use a grommet through every piece of sheet metal, and remember that sustained high sound pressure causes permanent hearing damage.