What size box for a 18 inch subwoofer?
Updated October 13, 2026
A 18 inch subwoofer usually needs 2.5–4 ft³ (71–113 L) in a sealed box or 4–6 ft³ (113–170 L) in a ported box tuned to about 27–32 Hz. Those are net (internal) volumes per sub, after the driver, port and bracing are taken out.
18 inch sub box size chart
| Sealed | Ported | Port tuning | |
|---|---|---|---|
| One 18″ sub | 2.5–4 ft³ (71–113 L) | 4–6 ft³ (113–170 L) | 27–32 Hz |
| Two 18″ subs | 5–8 ft³ (142–227 L) | 8–12 ft³ (227–340 L) | 27–32 Hz |
The chart shows the range most manufacturers print for 18 inch car subwoofers. It is a starting point, not an answer: two 18s with different Fs, Qts and Vas can want boxes that differ by a factor of two. If your sub's manual gives a recommended box, start from that.
What decides the exact volume
- Thiele/Small parameters. Vas (equivalent air volume) and Qts (total Q) set the sealed volume: a higher Vas or a lower Qts means a bigger box for the same response. Fs (free-air resonance) mostly sets how low a ported box can usefully be tuned.
- Sealed or ported. A ported box is usually 1.5–2 times the sealed volume for the same sub. See sealed vs ported.
- The car. The cabin lifts the low bass, so car boxes are often a little smaller (sealed) or tuned a little higher (ported) than the same sub would get in a home.
- Net vs gross. The numbers above are net. The outside size of the box is larger by the wood, the driver displacement and the port — see net vs gross volume.
Box depth for a 18 inch sub
We checked the published mounting depth of 206 18 inch subwoofers from 37 brands in our database. The typical (median) 18″ sub needs 11.25″ (286 mm) of mounting depth, and 1 in 10 need 13.25″ (335 mm) or more. Add about an inch for air behind the magnet and for cone travel, then choose width and height to reach the volume.
Short on depth — under a seat or behind a truck seat? Look for a shallow-mount 18″ sub; they are designed for small, shallow sealed boxes. More in under-seat and truck boxes.
Port size for a ported 18″ box
A common car-audio rule of thumb is 12–16 square inches of port area per cubic foot of net volume. For a single 18 that is often one 6–8″ round port, or a slot port of similar area. A port that is too small chuffs (air noise) at high power; the calculator warns when peak air speed in the port goes above about 66 ft/s (20 m/s). How to get the length right: how to calculate a ported box.
Get the exact numbers for your sub
Pick your 18 inch subwoofer from the database of about 2,000 models (or type its Fs, Qts and Vas), set the space you have, and the calculator gives the net and gross volume, port length, the response in your car, and the plans and cut sheet. It is free and works in inches or millimeters.
FAQ
What size box do I need for a 18 inch subwoofer?
Most 18 inch car subwoofers are designed for about 2.5–4 ft³ (71–113 L) sealed or 4–6 ft³ (113–170 L) ported, tuned around 27–32 Hz. The right volume for your sub depends on its Thiele/Small parameters, so check its manual or enter the model in the calculator.
How big of a box for two 18 inch subs?
Double the volume of a single sub: about 5–8 ft³ (142–227 L) sealed or 8–12 ft³ (227–340 L) ported for two 18s, plus the space taken by both drivers and the port.
How deep should a 18 inch sub box be?
Inside depth must be greater than the sub's mounting depth. Across 206 18 inch models in our database the typical mounting depth is about 11.25″ and 1 in 10 need 13.25″ or more, so plan for at least 12.25″ inside, then size width and height for the volume.