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

Subwoofer Box Dimensions Calculator

Quick answer

Internal volume is the external dimensions less twice the panel thickness on each axis. A 30 by 15 by 14 inch box in 3/4 inch MDF holds about 2.85 cubic feet gross, not the 3.65 the outside dimensions suggest.

Result

Gross internal volume 2.783 cu ft Published figure

In cubic inches 4809 cu in

Internal dimensions 28.5 x 13.5 x 12.5 in

Volume lost to panels 1491 cu in

Subtract the driver displacement, the port displacement and any bracing from this to get the net volume the driver actually sees.

The formula

Internal volume = (W - 2t) x (H - 2t) x (D - 2t), divided by 1,728 for cubic feet

Panel thickness costs more volume than people expect, and it costs it on both sides of all three axes. This is gross internal volume: the driver, the port and any bracing still have to come out of it.

The figures above are computed with these exact inputs: External width 30 in, External height 15 in, External depth 14 in, Panel thickness 0.75 in.

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.

Beginner

Goldwood Sound, Inc.

Goldwood TR12S sealed 12

$48.99

The cheapest sealed 12 here, and sealed is the alignment that forgives a volume error.

Check price on Amazon

Beginner

Q Power

QPower HD112 sealed 12

$49.99

A heavy duty single 12 sealed box built for a driver that is going to get used hard.

Check price on Amazon

Intermediate

Rockville

Rockville RMS12, 1.6 cu ft

$54.95

A stated 1.6 cubic feet in 3/4 inch MDF, which is at the large end of sealed for a 12 and suits a low Qts driver.

Check price on Amazon

Intermediate

BELVA

Belva MDFS12, 1.0 cu ft

$69.99

A stated 1.0 cubic feet, which is the most common published sealed recommendation for a 12 inch driver.

Check price on Amazon

Expert

Atrend

Atrend Bbox single sealed 12

$72.05

Better finish and bracing than the budget boxes, with terminals that will take real wire.

Check price on Amazon

Frequently asked questions

How does the box dimensions calculation work?: Internal volume = (W - 2t) x (H - 2t) x (D - 2t), divided by 1,728 for cubic feet. Panel thickness costs more volume than people expect, and it costs it on both sides of all three axes. This is gross internal volume: the driver, the port and any bracing still have to come out of it.
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.