Net vs gross subwoofer box volume
Updated October 13, 2026
Gross volume is calculated from the outside dimensions of the box. Net volume (airspace) is the air the subwoofer actually works with: the inside volume minus the sub, the port and any bracing. Every recommendation from a manufacturer is net.
How to calculate box volume in cubic feet
From outside size to net volume
- Gross: outside width × height × depth.
- Internal: subtract two wall thicknesses from each dimension. With ¾″ MDF that is 1.5″ off each one; with a double-thickness baffle subtract that too.
- Net: subtract what sits inside the box — the subwoofer's displacement (from its manual; roughly 0.05–0.15 ft³ for a 12″), the port (tube or slot, including the slot walls) and bracing.
Example
A box measuring 30 × 15 × 15″ outside, built from ¾″ MDF, with a 12″ sub and a 4″ round port 9.4″ long:
| Step | Calculation | Volume |
|---|---|---|
| Gross | 30 × 15 × 15 / 1728 | 3.91 ft³ (110.6 L) |
| Internal | 28.5 × 13.5 × 13.5 / 1728 | 3.01 ft³ (85.1 L) |
| Sub displacement | from the manual, here ~0.11 ft³ | − 0.11 ft³ |
| Port tube | 4¼″ outside diameter × 9.4″ | − 0.08 ft³ |
| Net | ≈ 2.82 ft³ (79.7 L) |
Almost a cubic foot separates the outside and the useful volume here. Building to the gross number is the most common reason a home-built box sounds different from the design.
Why it matters
- A sealed box that is 20 % too small raises Qtc and the bass gets peakier and less deep.
- In a ported box, missing volume raises the tuning frequency — you designed for 33 Hz and built 36 Hz.
More on what happens when the volume is off: box too small or too big.
Skip the arithmetic
Set the volume you want and the space you have; the calculator works out outside dimensions for the target net volume, accounting for material thickness, the sub, the port and braces, and shows both numbers in cubic feet and liters.