The speaker box volume calculator above answers the question that trips up almost every first enclosure build: the volume that matters is not the volume of the box you drew. It is the volume of air left inside once you have taken out the thickness of six panels, the basket and magnet of the driver, the tube of the port, and every brace you glued in. That last number is the one a driver responds to, and it is routinely ten to fifteen per cent smaller than the figure on the cutting plan.
Arb Digital builds free calculators that show their arithmetic rather than hiding it behind a single confident number. Here that matters more than usual, because the deductions are the whole story. A tool that quietly ignores driver and bracing displacement will hand you a box that measures right on paper and sounds wrong in the room.
What This Speaker Box Volume Calculator Does
It does two separate jobs and keeps them visibly separate. The first is pure woodwork geometry. You give it the outside width, height and depth of the finished cabinet and the thickness of the panel material, and it subtracts twice the thickness from each axis to get the internal cavity, then multiplies the three internal dimensions together. That produces the gross internal volume, expressed in litres, cubic feet and cubic inches, because enclosure work runs on all three depending on which side of the Atlantic the plans came from.
The second job is the deduction pass. Driver displacement, port displacement and bracing displacement are three separate fields rather than one lumped fudge factor, because they come from three different places: the driver's data sheet, your own port dimensions, and your own cut list. Subtracting them gives the net internal volume — the number you would compare against a design target.
An optional alignment check sits underneath. Enter the driver's Vas, Qts and Fs from the manufacturer's published data and a target Qtc, and the tool reports both the sealed box volume that target implies and the Qtc your box as drawn will actually deliver. That comparison is the fastest way to find out that a cabinet is twice the size it needs to be before you cut anything.
How to Use It
- Pick your unit, then enter the outside width, height and depth of the finished cabinet and the thickness of the panel stock. Use the real thickness — nominal 18 mm MDF is usually 18 mm, but nominal three-quarter-inch plywood is very often 18 mm rather than 19.05 mm.
- Read the gross internal volume. This is the empty cavity with nothing in it.
- Enter driver displacement from the data sheet. If the maker does not publish it, measure it: submerge the driver in a tub of water and read the rise, or fill the mounting recess and basket volume with dry rice and measure that.
- Enter port displacement. For a round vent it is simply the tube's outside cross-sectional area multiplied by the length inside the box, and the companion speaker port length calculator gives you that length for a chosen tuning.
- Add bracing and hardware, then read the net internal volume. If you are also checking an alignment, enter the three driver parameters and a target Qtc.
The Formula and How It Is Calculated
The geometry is straightforward. Internal width equals outside width minus twice the panel thickness, and the same for height and depth. Gross internal volume is the product of the three. Net internal volume is that figure minus driver, port and bracing displacement.
Work through the defaults. A 400 by 600 by 350 mm cabinet in 18 mm material has internal dimensions of 364 by 564 by 314 mm. That is 64,462,944 cubic millimetres, or 64.46 litres gross, which is 2.276 cubic feet. Subtracting 1.2 litres of driver, 0.9 litres of port and 0.5 litres of bracing leaves 61.86 litres net, or 2.185 cubic feet. The deductions cost 2.6 litres — four per cent here, and far more in a small monitor cabinet where the same driver eats a much larger share.
The sealed alignment uses the standard closed-box relations. The volume required for a target Qtc is Vb = Vas ÷ ((Qtc ÷ Qts)² − 1), and the system resonance is Fc = Fs × Qtc ÷ Qts. Running that backwards for a box you have already sized gives Qtc = Qts × √(Vas ÷ Vb + 1).
With the default driver — Vas 60 litres, Qts 0.42, Fs 32 Hz — a maximally flat Qtc of 0.707 needs 60 ÷ ((0.707 ÷ 0.42)² − 1) = 32.7 litres, with Fc at 53.9 Hz. The 61.86 litre box we just measured is nearly twice that, and delivers Qtc = 0.42 × √(60 ÷ 61.86 + 1) = 0.589 with Fc at 44.9 Hz. That is a legitimate, slightly overdamped alignment with a lower knee — not a mistake, but a different design, and worth knowing before the glue goes on.
Which Alignment This Page Implements, and Why It Says So
The alignment check here is the sealed (closed box) Qtc alignment and nothing else. It is not a vented Thiele-Small alignment, it does not compute an F3 for a ported box, and it makes no attempt at bandpass or passive radiator loading. Stating that plainly matters, because the same three driver parameters feed several different design procedures that give completely different volumes for the same driver.
The parameters themselves come from Neville Thiele and Richard Small's work formalising loudspeaker low-frequency behaviour. MONACOR's reference page on Thiele-Small parameters sets out what each one means and gives the widely used rule of thumb that a Qts below roughly 0.4 leans toward a vented enclosure, 0.4 to 0.7 toward a sealed one, and above 0.7 toward very large boxes or open baffles. PUI Audio's practical guide to sealed speaker enclosure design for small speakers describes the same workflow this page automates: target a system Qtc near 0.707, size for the required net back volume, then add the driver's displacement to get the enclosure you actually build.
That last step is why this tool works the way it does. The design procedure produces a net volume requirement. Your cutting plan produces a gross volume. The displacement fields are the bridge between them, and skipping them is the single most common way a carefully calculated box ends up wrong.
Why Published Driver Parameters Are Inputs, Not a Built-In Table
This page ships no driver database, and that is deliberate. Vas, Qts and Fs are measured quantities, not catalogue constants. They vary between production revisions of the same model number, they vary sample to sample within a batch, and they shift measurably once a driver has been broken in — the suspension loosens, Vas rises and Fs falls, sometimes by ten per cent or more in the first hours of use.
A built-in table would also freeze a snapshot of manufacturer data that gets revised without notice. If a page tells you a driver's Vas is 60 litres and the current data sheet says 52, the page is worse than useless because it looks authoritative. Take the numbers from the maker's own current data sheet for your exact model, and if you have measured your own pair, use those instead.
One practical consequence: if the two drivers in a stereo pair measure differently enough to matter, the honest move is to design for the average and accept a small mismatch, not to build two different boxes.
Where the Volume Actually Goes
The four deductions people underestimate, in rough order of how often they get missed. First, the driver itself. A modest six-and-a-half-inch woofer displaces something like half a litre; a large motor structure on a ten-inch subwoofer driver can pass two litres comfortably. In a five-litre bookshelf cabinet that is not a rounding error, it is ten to forty per cent of the design.
Second, the port. A vent tuned low is long, and a long vent inside a small box is a substantial object. The tube's own wall thickness counts too, since it is the outside volume of the tube inside the cavity that displaces air, not the bore.
Third, bracing. A single window brace across the middle of a cabinet, a shelf brace, and a few dowel braces can easily reach a litre together. Fourth, everything else: the terminal cup body, a crossover board mounted internally, and the mounting recess for the driver, which adds a little volume back rather than removing it.
Damping material is the odd one out. Stuffing does not simply displace air. Fibrous fill slows the propagation of sound inside the box and makes a sealed enclosure behave as though it were slightly larger — commonly cited as an apparent increase of up to about twenty per cent at high fill densities. This calculator does not model that, because the effect depends on the material and packing density in ways a single field cannot capture honestly. If you stuff heavily, expect the real acoustic behaviour to sit a little below the Qtc shown here.
Panel Thickness, Real Sheet Sizes and the Rounding Trap
Two thickness mistakes account for most of the errors people report. The first is using nominal thickness instead of actual. Sheet goods are frequently under their stated size, and the difference is doubled on every axis. A box built from stock that is 17.5 mm rather than 18 mm gains about a litre on the default cabinet above — small in absolute terms, meaningful in a compact design.
The second is mixing a doubled baffle into a single thickness figure. If the front panel is two layers of 18 mm and the rest is one, the tool's symmetric subtraction is wrong for the depth axis. The clean fix is to enter the single thickness and add the extra baffle layer's internal volume as part of the bracing displacement. That keeps the geometry honest instead of averaging two different panels into a number that describes neither.
If you are also working out how many sheets the cut list needs, our plywood sheet calculator handles the material side, and the board foot calculator covers solid timber for bracing and plinths. For converting a finished figure between litres, cubic feet and cubic inches on its own, the volume converter is quicker than doing it here.
How This Differs From the Adjacent Tools
Two live tools sit near this one without overlapping it. The speaker port length calculator takes the net volume this page produces and turns it into a vent length for a chosen tuning frequency — it sizes the vent, not the box. The aquarium glass thickness calculator shares the idea of a panelled box but answers a structural question about pressure and deflection rather than an acoustic one about enclosed air.
For the electrical side of a build, the Ohm's law calculator and the resistor power rating calculator cover crossover component sizing, and the decibel calculator handles level arithmetic once the cabinet exists.
Arb Digital designs and builds free interactive calculators that state their sources, show their arithmetic and earn links because they are genuinely useful. Browse what we have already published, or tell us what your audience keeps searching for.
Browse the Free Tools Hub Talk to Arb DigitalCommon Mistakes to Avoid
- Designing to gross internal volume. The alignment maths wants the net figure, after the driver, port and bracing have been taken out. Gross is the cavity; net is what the driver sees.
- Subtracting thickness once per axis. Every dimension has a panel at both ends, so thickness comes off twice — 36 mm from each axis for 18 mm stock.
- Using catalogue driver parameters. Take Vas, Qts and Fs from the current data sheet for your exact model, and remember they drift as the suspension breaks in.
- Forgetting the port tube's own volume. A low tuning needs a long vent, and a long vent inside a small box is one of the larger deductions on the list.
- Assuming stuffing is just displacement. Fill changes the apparent size of a sealed box rather than simply removing air, and this page deliberately does not model that.
Related Free Tools From Arb Digital
Pair this with the speaker port length calculator for vented designs, the plywood sheet calculator and board foot calculator for materials, the volume converter for unit changes, and the Ohm's law calculator plus decibel calculator for the electrical and level side. Everything else is on the free online tools hub.
Frequently Asked Questions
Inside, and after deductions. The figure that matters acoustically is the net internal volume: the cavity left once panel thickness has been subtracted twice from each axis and the driver, port and bracing have been taken out.
It depends entirely on the motor and basket, so take it from the manufacturer's data sheet. As a rough sense of scale, a small mid-bass driver may displace well under half a litre while a large subwoofer motor can pass two litres. If the figure is not published, measure it by water or dry rice displacement.
The optional check is a sealed, closed-box Qtc alignment only. It uses Vb = Vas divided by ((Qtc/Qts) squared minus one) and Fc = Fs times Qtc divided by Qts. It is not a vented Thiele-Small alignment and does not model bandpass or passive radiator loading.
0.707 is the maximally flat alignment and the usual starting point. Lower values are more damped with a gentler roll-off and better transient behaviour in room, higher values give a peak before the knee. The right answer depends on the driver, the room and your preference, which is why it is an input here rather than a fixed constant.
No, and treating it that way is misleading. Fibrous fill slows sound propagation inside the box and makes a sealed enclosure behave as though it were somewhat larger. The effect depends on the material and packing density, so this calculator leaves it out rather than guessing at it.
Because published parameters change between production revisions, vary from sample to sample, and shift as a driver breaks in. A frozen table would look authoritative while being wrong for your specific driver, so the three parameters are inputs you take from the current data sheet.
This page sizes the box: internal cavity, deductions and net volume, plus an optional sealed alignment check. The port length calculator takes a net volume as given and works out how long the vent must be to tune the box to a chosen frequency. They are consecutive steps, not alternatives.
This is a design aid, not a substitute for measurement. Enclosure volume is only one of several factors that determine how a loudspeaker performs, published driver parameters carry real tolerances, and the alignment shown here assumes an ideal sealed box with no leakage.
