How do I calculate cubic feet of a subwoofer enclosure?

Multiply interior length × width × height in inches and divide by 1,728. Subtract the volume taken by the driver, bracing, ports, and any internal stuffing for the "net" volume manufacturers list.

What is driver displacement?

Driver displacement is the volume of the speaker basket, motor, and cone that intrudes into the enclosure. Manufacturers publish a "displaced volume" figure (typically 0.05–0.20 ft³) that you subtract from gross internal volume.

Should I use gross or net cubic feet?

Net. Manufacturer spec sheets list recommended sealed/ported volume as net interior — after driver, bracing, and port displacement. Designing to gross overshoots volume by 5–15% and changes box tuning.

How much volume does a port take?

A port the size of a 3-inch ID × 12-inch long PVC pipe displaces about 85 in³ ≈ 0.05 ft³. Multiply π × radius² × length, in inches, divided by 1,728.

Subwoofer Cubic Feet Calculator — Net Internal Volume for Sub & Speaker Box

How to calculate cubic feet for subwoofer box

Subwoofer enclosure design is sensitive to internal volume — even a 10% deviation shifts box tuning enough to color the sound. Calculate gross internal cubic feet by measuring interior length × width × height (in inches) and dividing by 1,728.

Then subtract:

Driver displacement: 0.05–0.20 ft³ depending on driver size and motor
Port displacement: 0.04–0.10 ft³ for typical 3-inch round ports
Internal bracing: 0.02–0.05 ft³ for cross-bracing on large boxes

Worked examples

Example 1: A 24 × 14 × 12 inch sealed box for a 12-inch sub

Internal: 24 × 14 × 12 = 4,032 in³ ÷ 1,728 ≈ 2.33 ft³ gross. Subtract 0.15 ft³ driver displacement: 2.18 ft³ net.

Example 2: A 30 × 18 × 16 inch ported box

Internal: 30 × 18 × 16 = 8,640 in³ ÷ 1,728 = 5.0 ft³ gross. Subtract 0.20 ft³ for driver and 0.10 ft³ for port: 4.70 ft³ net.

Example 3: A 16 × 16 × 16 inch cube for a 10-inch sub

16 × 16 × 16 = 4,096 in³ ÷ 1,728 ≈ 2.37 ft³ gross. Subtract 0.10 ft³ for the driver: 2.27 ft³ net.

Common driver displacement

Driver size	Conversion factor	Worked example
8 inch sub	0.04–0.07 ft³	Sealed: 0.5–0.75 ft³
10 inch sub	0.07–0.12 ft³	Sealed: 0.75–1.25 ft³
12 inch sub	0.10–0.18 ft³	Sealed: 1.25–2.0 ft³
15 inch sub	0.18–0.30 ft³	Sealed: 2.5–4.0 ft³
18 inch sub	0.30–0.50 ft³	Sealed: 4.5–7.0 ft³

Tips and considerations

Use 3/4 inch material

MDF or birch plywood at 3/4 inch is the standard. Subtract 1.5 inches (two walls) from each external dimension to get the interior.

Brace large boxes

Boxes over 2 ft³ benefit from cross-bracing. The brace itself displaces 0.02–0.05 ft³ — small enough to ignore for sealed designs but worth subtracting for tightly-tuned ported boxes.

Common mistakes to avoid

Using external dimensions for internal volume. A box is built from MDF that is typically 3/4 inch thick. Subtract twice the panel thickness from each external dimension before calculating internal volume.
Forgetting driver displacement. The subwoofer itself takes up 0.05–0.15 ft³ inside the box. Subtract the manufacturer's displacement spec from your gross internal volume.
Forgetting bracing and port volume. Internal bracing typically costs 0.02–0.05 ft³. Ports add another 0.05–0.20 ft³ depending on tuning. Subtract both.
Mismatching the spec sheet. Driver specs list a recommended net internal volume. Match your final number (after all subtractions) to that spec, not the gross box volume.

Related concepts and calculators

Speaker enclosure design overlaps with a couple of calculator pages:

Wood and lumber calculator — for figuring out board feet from MDF or plywood cuts.
Cubic inches to cubic feet — most driver displacement specs are in cubic inches.
Rectangular calculator — for verifying total external box dimensions.
Cylinder calculator — for port-tube volume calculations.