Speaker box volume
net volume after driver & port

Enter the cabinet’s outside size and panel thickness. Get the gross internal volume, then the net volume the driver actually sees once the driver, port and bracing are taken out.

Enclosure, outside
Port
Bracing, fill and a target tuning

Polyfill damping is not a displacement — it makes the box behave as if it were slightly larger, so leave it out of this figure. Count braces, ports and anything else solid.

How it works

Gross, then net

Gross internal volume is the outside box less two panel thicknesses on each axis: (W − 2t) × (D − 2t) × (H − 2t). Net volume is what the driver actually sees, so everything solid inside comes off it — the part of the driver that sits inside the box, the port tube, and the braces. Manufacturers publish the first figure as the driver’s displacement (sometimes “net volume occupied”); it is typically well under a litre for a small mid and a couple of litres for a big woofer, but read it off the spec sheet rather than guessing. A port displaces its own outside volume; this tool uses the inside cross-section × length, which is close enough for a thin plastic tube and a little low for a thick timber slot.

Polyfill is deliberately not subtracted. Loose fill slows the air inside and makes a sealed box measure as though it were up to about 20% larger, which is the opposite of a displacement.

The tuning figure

Where the Hz number comes from

A vented box and its port form a Helmholtz resonator, so the tuning frequency is

Fb = (c / 2π) × √( A / (V × Leff) )

with c the speed of sound, taken as 343 m/s at 20 °C, A the total inside cross-section of the ports, V the net volume from the table above, and Leff the port length plus an end correction. The end correction here is 0.732 × the port diameter — the textbook 0.85 × radius for the flanged end on the baffle plus 0.61 × radius for the free end inside the box. A slot port is converted to the diameter of a circle with the same area.

This is arithmetic from a formula, not a published specification, and it assumes a rigid box, a port that doesn’t touch the back wall and an enclosure that is not stuffed. Treat it as a starting length: build the box, sweep it, and take the tuning from the impedance minimum between the two peaks. A port ending within about its own diameter of a wall tunes lower than this predicts.

FAQ

Questions

How do I calculate the internal volume of a speaker box?
Subtract twice the panel thickness from each outside dimension, then multiply: (W − 2t) × (D − 2t) × (H − 2t). That is the gross internal volume. Net volume is that figure minus the driver displacement, the port’s volume and any bracing.
Do I subtract the driver from the box volume?
Yes — the part of the driver inside the box takes up air the driver would otherwise see. Use the displacement figure from the driver’s datasheet, not the frame diameter. Passive radiators and amplifier plates count too.
Does polyfill count as displacement?
No. Damping material doesn’t take volume away; it slows the air inside so a sealed box behaves as if it were larger — by up to roughly 20% when well stuffed. Leave it out of the displacement figure.
What is the formula for port tuning frequency?
The Helmholtz resonance: Fb = (c / 2π) × √(A / (V × Leff)), where c is 343 m/s, A is the port’s inside area, V is the net internal volume and Leff is the port length plus an end correction of about 0.732 × the port diameter. Measure the finished box to confirm it.
Is this the same as the shipping size?
No. This page works inward from the outside of the cabinet; carriers charge on the outside of the shipping carton, which is larger again. The result links through to the dimensional weight for the cabinet’s outside size.