What box should you build — and will it fit?

Pick your vehicle and a real subwoofer. You get a sealed or ported box with real external dimensions, an honest fit-check against your actual cargo space, and a build checklist and cut list. Every number says where it came from.

Don't want to compute your own box? Browse pre-made flat-packs by sub size instead: Down4Sound PRO-FAB or Next Level Fabrications (both sell fixed-size catalog boxes, not vehicle-matched — good if you just want "a decent box for a 12," not an exact fit).

1. Vehicle

2. Driver (subwoofer)

3. Box type

why?
0.707 is the flat, daily-driver default — it is the Butterworth alignment, the Qtc that gives the flattest response with no peak before rolloff, and the same default this app uses in the bandpass estimator. Lower (0.6-0.65) leans tight/accurate, higher (0.8+) leans boomy. This is a design choice, not a fixed constant — override it if you want a different tune.

4. External footprint & materials

The target net volume from step 3 can be built in many shapes — one volume maps to infinite width/height/depth combinations. Pick a width and height that fit your vehicle's cavity; depth is solved to hit the target volume exactly.

why?
Changes which panel gets the driver cutout on the cut-sheet, and which dimension has to clear the driver's mounting depth. A downfire box in a truck under-seat space is short and wide — the driver goes in the bottom, so its magnet needs the box's height, not its depth. Default is front, this tool's original behaviour.
Defaults to 0.75in — standard 3/4" MDF, the baseline material per the enclosure design knowledge base. Measure your sheet and enter what you measure: plywood sold as 3/4" is 23/32" (0.72").
why?
45° fillets on every internal 90° corner are our standing shop practice, and the published guides call for a strip or bead on every inside seam for strength and an airtight seal. Where air moves fast — the port — rounded edges also cut turbulence. They are also wood inside the box. Solving them in makes the box bigger so the airspace still lands on target; leaving them out keeps this link's dimensions unchanged and the plan tells you what they'll cost instead. On a 1.6 ft³ build a 1/2" leg is about 0.8% of the airspace and a 3/4" leg about 1.7%; the cost grows with the square of the leg, so 1 1/2" is roughly four times 3/4".
why?
A double baffle stiffens the panel the driver hammers hardest and gives screws more to bite. It is also wood inside the box: each extra layer takes its thickness off the airspace, so the box is solved bigger to keep the same net volume. On a 1.6 ft³ build one extra 3/4" layer is about 0.125 ft³ — nearly 8%.
why?
Flush is the stronger build and the default. Every layer is cut to the same through-hole, so the driver's flange bears on the whole laminated stack and its screws bite the full thickness. That matters most with a heavy driver — a big 18 can run near a hundred pounds, and you want it landing on doubled or tripled material, not on a shelf routed into one sheet.

Recessed cuts the outer layer to the driver's flange diameter and the inner layer to the through-hole, so the driver drops in level with the face. It is a cosmetic choice, it needs a published flange diameter (142 of our 169 drivers have one), and the cut sheet will refuse it — and say why, in inches — if the recesses would run into each other or off the edge of the panel.
why?
Braces and seam cleats are wood inside the box, so their volume has to come out of the airspace. Unlike the fillets and baffle layers above, the size here is genuinely your call — three cross-sections are on the plan and the right one depends on the span and whether it has to clear the magnet — so this stays a number you enter rather than one we pick. Build once with 0, then read the "construction displacement" table on the printed plan: it gives you the exact figure to put here. Re-run and the depth is solved with it in. See the Net Volume Helper for the estimating method.
Pick a vehicle and a driver, then hit Build.