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CarAudioCalc

Enclosure calculator

Net vs Gross Box Volume Calculator

Quick answer

Net volume is gross internal volume less everything inside the box. A 2.85 cubic foot gross box with a 0.12 cubic foot driver, a 0.30 cubic foot port and 0.05 of bracing has 2.38 cubic feet net, which is what the driver actually sees.

Your numbers

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Result

Net internal volume 2.38 cu ft Published figure
Gross internal volume 2.85 cu ft
Total displacement 0.47 cu ft
Bracing allowance 0.05 cu ft Convention

Every manufacturer enclosure recommendation is a net figure. Building to the gross number puts the driver in a box that is meaningfully too small.

The formula

Net = gross internal volume - driver displacement - port displacement - bracing

Driver displacement is a published figure on the specification sheet and cannot be derived from the nominal size. Without it this returns nothing rather than guessing, because a guess here is exactly the error that ruins a subwoofer.

The figures above are computed with these exact inputs: Gross internal volume 2.85 cu ft, Driver displacement 0.12 cu ft, Port displacement 0.3 cu ft, Bracing 0.05 cu ft.

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.

AssumptionKindWhere it comes from
Bracing Displacement: 0.05ConventionA planning allowance of roughly 0.05 cubic feet for internal bracing in a typical single-driver enclosure. Actual bracing displacement is whatever you build, and should be measured rather than assumed

Gear that matches this result

Every card below is in the page source. The calculator re-ranks them against your numbers and marks anything that falls short of what you just calculated.

Frequently asked questions

How does the net vs gross volume calculation work?
Net = gross internal volume - driver displacement - port displacement - bracing. Driver displacement is a published figure on the specification sheet and cannot be derived from the nominal size. Without it this returns nothing rather than guessing, because a guess here is exactly the error that ruins a subwoofer.
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.