Quick answer: subtract twice the wood thickness from each external dimension, multiply, divide by 1,728 for cubic feet — then subtract the driver, port and bracing displacement. That last step is what most people skip, and it is why boxes come out undersized.
Gross to net, step by step
Gross ft³ = internal in³ ÷ 1,728
Net = gross − driver − port − bracing
Worked example — a 30 × 15 × 15 inch box in 3/4 inch MDF with a 12-inch driver:
- Internal: 28.5 × 13.5 × 13.5 = 5,194 in³
- Gross: 5,194 ÷ 1,728 = 3.01 ft³
- Driver displacement: −0.15 ft³
- Net: 2.86 ft³
If the driver's sheet asked for 3.0 ft³ net, that box is slightly small — close enough to work, but it shows how quickly the margin disappears. Add a 4-inch port and you would lose another 0.10 ft³ on top.
Typical displacement figures
| Driver size | Displacement | Sealed box (typical) | Ported box (typical) |
|---|---|---|---|
| 8 in | 0.05–0.10 ft³ | 0.35–0.7 ft³ | 0.7–1.2 ft³ |
| 10 in | 0.08–0.15 ft³ | 0.6–1.0 ft³ | 1.0–1.75 ft³ |
| 12 in | 0.10–0.20 ft³ | 1.0–1.75 ft³ | 1.75–3.0 ft³ |
| 15 in | 0.15–0.30 ft³ | 1.75–3.0 ft³ | 3.0–5.0 ft³ |
| 18 in | 0.25–0.45 ft³ | 3.0–5.0 ft³ | 5.0–8.0 ft³ |
These are ranges, not recommendations. Always build to the volume in your driver's spec sheet — two 12-inch subs from different makers can want boxes that differ by more than a cubic foot, because enclosure volume is dictated by the driver's Thiele–Small parameters rather than its diameter.
Sealed or ported?
| Sealed | Ported | |
|---|---|---|
| Box size | Smaller | Larger, often double |
| Output near tuning | Lower | Noticeably higher |
| Roll-off | Gentle (12 dB/octave) | Steep below tuning (24 dB/octave) |
| Sound character | Tight, accurate | Loud, more impact |
| Build difficulty | Straightforward | Port length and tuning must be right |
| Best for | Music, tight spaces, accuracy | Bass-heavy music, maximum output |
Ported boxes also need care below tuning: with little air load to restrain it, the cone can over-excurse on very low content, so a subsonic filter is standard practice on a ported build.
Common mistakes & pro tips
- Using external dimensions. Three-quarter-inch MDF takes 1.5 inches off each dimension. On a small box that is a large fraction of the volume.
- Forgetting displacement. Driver, port and bracing together can eat 10% of a small enclosure.
- Sizing by driver diameter. Two 12s can need very different boxes. Read the spec sheet.
- Ignoring the port's own volume. The tube takes up space inside the box, and it counts.
- Pro tip — measure displacement by water. If the sheet does not list it, seal the driver in a bag, submerge it in a marked container, and read the displaced volume. That is exactly the water-displacement method for irregular objects.
- Pro tip — build slightly over. It is easy to reduce net volume by adding a block of wood inside; it is not easy to make a finished box bigger.
Frequently asked questions
How do I calculate subwoofer box volume?
Work from internal dimensions: subtract twice the wood thickness from each external measurement, multiply the three together, then divide by 1,728 for cubic feet. A 30 × 15 × 15 inch box built from 3/4 inch MDF has internal dimensions of 28.5 × 13.5 × 13.5, giving 5,194 in³ or about 3.0 cubic feet gross.
What is net versus gross box volume?
Gross is the raw internal space. Net is what the driver actually sees, after subtracting the volume displaced by the woofer's magnet and basket, the port tube, and any internal bracing. Manufacturers specify their recommended volume as net, so building to the gross figure leaves the box undersized.
How much volume does a subwoofer displace?
Roughly 0.05 to 0.1 cubic feet for an 8 inch driver, 0.08 to 0.15 for a 10 inch, 0.1 to 0.2 for a 12 inch, and 0.15 to 0.3 for a 15 inch. The exact figure appears in the driver's spec sheet — use that rather than an estimate, because high-excursion drivers with large motors displace far more than the range suggests.
Should I build a sealed or ported box?
Sealed boxes are smaller, simpler and give tight, accurate bass with a gentle roll-off — good for music and for anyone short on space. Ported boxes are larger and louder near the tuning frequency, which suits bass-heavy music, but they roll off steeply below tuning and are less forgiving of design errors.
Does port volume count against box volume?
Yes. The port tube occupies internal space, so its outside volume must be subtracted from the gross to get net. A 4 inch diameter port 14 inches long displaces about 176 in³, roughly 0.10 cubic feet — enough to matter in a small enclosure.