Speaker Box Calculator

ENCLOSURE DESIGN TOOL

Calculate usable airspace, design box dimensions, or estimate a sealed enclosure from your driver’s specifications. Values update as you type.

Measurement system

Box type

Outside cabinet dimensions

Outside width in

Outside height in

Outside depth in

Wall thickness in

Internal displacement

Total driver displacement ft³

Bracing / other displacement ft³

Use manufacturer driver displacement. If unknown, leave blank; the calculation temporarily treats it as zero and flags the missing value. Do not double-count internal port volume.

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LIVE RESULTS

Net internal airspace

—

Outside dimensions: —
Inside dimensions: —
Gross internal volume: —
Usable net airspace: —
Occupied volume: —
Estimated physical port length: Not applicable
Predicted sealed system: Not applicable
Clear port area: Not applicable
Total occupied port volume: Not applicable
Port clearance: Not applicable

Dimension labels reflect calculated outside measurements. Schematic cabinet, not a scaled cutting drawing.

Panel cut list

Six-panel butt-joint box: front and back cover the full width and height; top/bottom fit between them; sides fit between front/back and top/bottom. No cutouts, bracing, port panels, saw kerf or joinery allowances included. Verify each panel and material thickness physically before cutting.

Rectangular panel dimensions
Panel	Qty	Cut size
Enter valid dimensions.

Check manufacturer enclosure recommendations and mounting clearances before cutting material. Acoustic models and end corrections are estimates.

Enclosure sizing guide

How to Use the Speaker Box Calculator

Use the speaker box calculator above to find a rectangular enclosure’s usable internal airspace, solve for an outside dimension from a target volume, or estimate a sealed-box volume from driver specifications. Enter outside dimensions and actual wall thickness, then account for the speaker, bracing and any modeled internal port. Net airspace—not the cabinet’s exterior volume—is the value to compare with your driver’s enclosure requirement.

Choose Check box volume to measure an existing cabinet, Design dimensions to size a proposed cabinet, or Sealed driver design to estimate an ideal sealed alignment from Fs, Vas and Qts. Select Imperial or Metric units before entering your measurements; values and results update as the inputs change.

Check an existing box: Outside width, height, depth and wall thickness → net airspace.
Design a new box: Target net airspace and two fixed outside dimensions → the missing outside dimension.
Acoustic estimates: Ideal sealed chamber sizing or approximate straight-port length for a specified volume and tuning.

Select the mode that matches what you know

Already know your cabinet dimensions?
Select Check box volume, enter the complete outside width, height and depth, then enter actual material thickness and total physical displacement. Read the net internal airspace result.
Already know the required net volume?
Select Design dimensions, enter the target, choose width, height or depth as the dimension to solve, and enter the other two outside dimensions and material thickness. Review the solved dimension and physical fit before cutting.
Have the driver’s Thiele–Small specifications?
Select Sealed driver design and supply Fs, Vas, Qts, target Qtc and the number of identical drivers sharing the chamber. This mode estimates ideal net sealed volume and system resonance. Its displayed exterior geometry additionally depends on the dimension-mode width, height and thickness values retained by the calculator; switch to Design dimensions to set and verify those construction dimensions explicitly.

Gross Volume, Net Airspace and Displacement

An enclosure’s outside dimensions include the cabinet walls. Gross internal volume measures the clear cavity inside those walls; net airspace is what remains after subtracting components occupying that cavity. Use the correct volume convention when comparing your answer with a manufacturer’s specified box volume.

Outside versus inside dimensions: For the calculator’s uniform-thickness rectangular shell, subtract twice the material thickness from each outside dimension. Measure the actual board thickness, not merely its nominal product name.
Gross internal volume: Inside width × inside height × inside depth, before displacement. The exterior wall material has already been excluded and must not be subtracted again.
Total physical driver displacement: The volume of all installed driver structures projecting into the modeled chamber. For several drivers, enter the combined occupied volume. Changing the number of drivers in the sealed-design inputs does not establish this separate physical displacement for you.
Bracing and other displacement: The part of internal braces, partitions and other components actually within the chamber, excluding any volume already counted in another field.
Modeled port assembly volume: For the calculator’s fully internal straight-port model, the port’s outer envelope is excluded from the main chamber’s net airspace. Do not enter that same modeled port volume again as bracing or other displacement.
Unknown displacement: A blank displacement field is treated as zero for the provisional calculation and flagged as unknown; it is not evidence that the installed component occupies no space. Replace provisional zero values before finalizing the design.

Read the manufacturer’s volume convention

A datasheet’s recommended enclosure figure may already account for the speaker’s occupied volume, or it may specify a gross construction volume that still needs adjustments. Read the exact model’s notes before applying an additional allowance. Do not confuse physical driver enclosure displacement with diaphragm displacement derived from cone area and excursion; they are different quantities.

Speaker Box Volume and Dimension Formulas

For a six-panel rectangular enclosure of uniform wall thickness, find the three clear internal dimensions, multiply them, and subtract the volume occupied by internal components. All terms in a single equation must use consistent units.

Clear internal dimensions

\[\begin{aligned}W_i&=W_o-2t\\H_i&=H_o-2t\\D_i&=D_o-2t\end{aligned}\]

Subtract one wall thickness at each end of each outside dimension. Every resulting clear dimension must be positive.

Gross and net airspace

\[\begin{aligned}V_g&=W_iH_iD_i\\V_n&=V_g-V_d-V_r-V_p\end{aligned}\]

Multiply the inside dimensions to obtain gross internal volume; subtract the total physical driver displacement, bracing/other displacement and applicable port-assembly displacement to obtain net airspace.

When inside dimensions are in inches, divide cubic inches by 1,728 for cubic feet. When dimensions are in millimeters, divide cubic millimeters by 1,000,000 for liters. One cubic foot equals 28.316846592 liters.

Solving for outside depth

\[D_o=\frac{V_n+V_d+V_r+V_p}{(W_o-2t)(H_o-2t)}+2t\]

Add the required net airspace to all occupied volumes, divide by the two fixed clear inside dimensions, then add both depth-wall thicknesses. With dimensions entered in inches, convert the total required volume to cubic inches before dividing by an area in square inches. Width and height are solved by the corresponding rearrangement.

For a ported design, the occupied port volume depends on its physical length and can change with the solved cabinet dimension. Use the calculator’s coupled port/volume result rather than assuming a fixed port allowance when the port geometry changes.

\(W_o,H_o,D_o\): Outside dimensionsFinished cabinet width, height and depth, respectively.in or mm
\(W_i,H_i,D_i\): Clear internal dimensionsCavity width, height and depth after subtracting the two relevant exterior walls.in or mm
\(t\): Uniform wall thicknessActual thickness of the modeled exterior panels.in or mm
\(V_g,V_n\): Gross internal volume and net airspaceFull clear cavity before displacement and remaining modeled air volume after displacement, respectively.Consistent volume units
\(V_d,V_r,V_p\): Occupied volumesTotal driver, bracing/other, and applicable internal port-assembly displacement, respectively. Every physical portion is counted once.Consistent volume units

Speaker Box Volume Calculation Example

Consider an illustrative sealed enclosure measuring 24 × 16 × 16 inches outside, built from 0.75-inch material. Assume the installed driver’s physical displacement is 0.10 ft³ and internal bracing occupies 0.05 ft³. These displacement figures are example inputs, not specifications for a particular driver.

Given values

Outside width × height × depth: 24 × 16 × 16 in
Uniform wall thickness: 0.75 in
Physical driver displacement: 0.10 ft³
Bracing displacement: 0.05 ft³
Port: None (sealed box)
Find: Gross volume and net airspace

Find the internal dimensions

\[\begin{aligned}W_i&=24-1.5=22.5\text{ in}\\H_i&=16-1.5=14.5\text{ in}\\D_i&=16-1.5=14.5\text{ in}\end{aligned}\]

The clear cavity is 22.5 × 14.5 × 14.5 inches.

Calculate gross internal airspace

\[\begin{aligned}V_g&=22.5(14.5)(14.5)\\&=4730.625\text{ in}^3\\&=\frac{4730.625}{1728}\text{ ft}^3\\&\approx2.7376302\text{ ft}^3\end{aligned}\]

Convert the cubic-inch cavity volume into cubic feet before subtracting the two displacement inputs.

Subtract internal displacement

\[V_n=2.7376302\ldots-0.10-0.05\approx2.5876302\text{ ft}^3\]

Driver and bracing together occupy 0.15 ft³ of the clear cavity.

Result

Net airspace: 2.588 ft³ (approximately 73.27 L)

This is the modeled usable air volume. It establishes geometric airspace, not whether this box suits a particular speaker.

How to Interpret Your Box Results

Compare the usable net airspace with the exact driver’s specified enclosure requirement. Review the clear internal dimensions and occupied-volume breakdown as separate checks: an apparently suitable net volume does not establish that the driver’s basket, magnet or port can physically fit.

Net airspace

This is the modeled chamber air volume after the listed displacement. If a needed displacement value is unknown, the displayed result is provisional rather than a final build dimension.

Dimensional sensitivity

With the other two clear internal dimensions and all displacement held fixed, a 10% increase in one clear dimension increases gross cavity volume by 10%. The percentage increase in net airspace may be larger because the occupied volume remains fixed.

Sanity checks

Gross internal volume must be below the outside rectangular envelope for positive wall thickness. With nonnegative occupied volumes, net airspace cannot exceed gross internal volume; zero or negative usable airspace is not a buildable result.

Choosing a Sealed or Ported Box Size

Choose the enclosure type and target volume from the specific driver’s manufacturer guidance or a supported acoustic design. Two nominally 12-inch drivers can require different cabinet volumes; diameter alone cannot determine the appropriate airspace or tuning.

Sealed enclosure

A sealed box uses a closed air chamber with no tuned vent. The calculator can evaluate an entered rectangular volume or estimate an ideal sealed volume from Fs, Vas, Qts and target Qtc. Compare the result with the intended driver design and physical space.

Ported enclosure

A ported box adds a tuned air passage. This calculator needs the net volume and desired tuning frequency as inputs to estimate straight round or rectangular port geometry; it does not select optimal volume or tuning for a given driver.

How should you size a box for two subwoofers?

If a manufacturer’s net airspace requirement is per driver, two identical drivers sharing one chamber ordinarily need the corresponding combined target under that design convention. Enter their combined physical displacement separately. For divided chambers, calculate each chamber’s clear net volume independently and include the divider’s actual occupied volume when determining the cabinet shell.

Sealed Speaker Box Design from Driver Parameters

The sealed-driver mode uses ideal small-signal relationships to estimate a net chamber volume at a chosen system Qtc. It requires the driver’s free-air resonant frequency (Fs), equivalent compliance volume (Vas), total free-air quality factor (Qts), and the number of identical drivers sharing the chamber.

Ideal sealed net volume

\[V_s=\frac{NV_{as}}{(Q_{tc}/Q_{ts})^2-1}\]

Multiply the per-driver Vas by the number of identical drivers and divide by the squared system-Q to driver-Q ratio minus one. Vas and the result must use matching volume units.

A finite positive answer requires Qtc greater than Qts. As the target Qtc approaches Qts from above, the calculated required volume increases without bound.

Ideal system resonance

\[F_c=F_s\frac{Q_{tc}}{Q_{ts}}\]

The ideal system resonant frequency equals free-air resonance multiplied by the ratio of target Qtc to driver Qts. Fc is not automatically the system’s −3 dB frequency, nor a measured in-vehicle or in-room response.

\(V_s\): Required net sealed-chamber volumeAirspace predicted for the selected ideal alignment.ft³ or L
\(N\): Identical drivers sharing one chamberWhole-number driver count under the common-chamber model.Count
\(V_{as}\): Equivalent compliance volume per driverManufacturer’s acoustic compliance parameter; not physical driver displacement.ft³ or L
\(Q_{tc},Q_{ts}\): Target sealed-system Q and free-air driver QDimensionless damping parameters. The target Qtc must exceed Qts for a finite positive modeled box volume.Dimensionless
\(F_c,F_s\): Sealed-system and free-air resonancePredicted closed-box resonance and the input driver’s free-air resonance, respectively.Hz

Why box size changes a sealed system

With driver parameters held fixed, reducing chamber volume increases the modeled stiffness contribution of the enclosed air, increasing Fc and Qtc. A larger chamber moves the ideal system behavior toward the driver’s free-air parameters. Real cabinet leakage, damping and installation effects are not represented by this simple alignment calculation.

After choosing an ideal acoustic net volume, check its exterior dimensions using Design dimensions mode. The current sealed-driver display also uses retained construction dimensions from the dimension-design inputs; do not treat those displayed cabinet dimensions as an independently optimized shape.

Ported Box Tuning and Port Length

For a ported enclosure, enter the intended net chamber volume, target tuning frequency Fb, port shape, clear opening dimensions, number of identical ports and an approximate termination condition. The calculator estimates the physical length of each identical straight port; the displayed length is not the sum of all port lengths.

Ideal Helmholtz relationship

\[F_b=\frac{c}{2\pi}\sqrt{\frac{S}{V_nL_{\mathrm{eff}}}}\]

Tuning frequency depends on sound speed, total clear port area, net enclosure air volume and effective acoustic port length. Use meters, square meters, cubic meters and meters per second consistently in this equation.

Round or rectangular opening area

\[\begin{aligned}A_{\mathrm{round}}&=\frac{\pi d^2}{4}\\A_{\mathrm{slot}}&=wh\\S&=nA\end{aligned}\]

For one round port, use the clear inside diameter—not the outside tube diameter. For a straight rectangular slot, multiply the clear opening width by its clear height. Multiply one port’s area by the number of identical ports for total area.

\(F_b\): Box tuning frequencySpecified target Helmholtz tuning.Hz
\(c\): Speed of soundSound speed assumed by the ideal port model.m/s
\(S,A,n\): Total area, one-port area, and port countTotal clear port area, clear area of one identical opening, and number of identical openings.m²; m²; count
\(L_{\mathrm{eff}}\): Effective acoustic port lengthPhysical length plus the modeled acoustic end correction.m
\(d,w,h\): Clear port dimensionsRound inside diameter, rectangular opening width, and rectangular opening height.m

What changes the actual length and tuning?

The acoustic end correction makes effective length different from the physical length you cut. The selected flanged/unflanged option is an approximate model; flares, a wall-integrated slot, a folded passage, nearby internal surfaces and obstructions can change actual tuning. The rectangular-slot end correction is a circular-equivalent approximation, not a geometry-specific prediction.

For fixed net airspace and Fb, increasing clear port area increases the ideal required effective length. In a fixed cabinet, increasing the length of a fully internal port also increases the volume occupied by its outer envelope, leaving less main-chamber airspace. The net chamber volume and port length therefore need to be solved together, not treated as unrelated measurements.

The calculator’s displacement model assumes that each complete straight port assembly lies inside the box; a slot is modeled with four independent internal walls. An exterior port or a slot sharing existing enclosure walls requires different physical volume accounting. Do not add the calculator’s already modeled port displacement a second time as bracing.

Panel Cut List and Pre-Build Checks

The six-panel cut list applies to the calculator’s specific rectangular butt-joint arrangement: full-width, full-height front and back panels; top and bottom fitted between the front and back; and side panels fitted between front/back and top/bottom. A different overlap convention changes the required cut sizes even if the finished outside envelope is unchanged.

Cut dimensions for the calculator’s six-panel shell
Panel	Qty	Cut dimensions
Front and back	2	Outside width × outside height
Top and bottom	2	Outside width × clear internal depth
Left and right sides	2	Clear internal height × clear internal depth

The cut list describes the exterior shell only. It does not provide driver mounting cutouts, screw-pattern locations, bracing cuts, port panels, saw-kerf allowances or alternate joinery. In ported mode, the displayed box panel list is not a complete port-panel cutting schedule.

Verify these measurements before cutting

Check the exact driver model’s intended net sealed or ported enclosure volume and the manufacturer’s displacement convention.
Measure the actual board thickness and recalculate if it differs from the value entered.
Confirm that total physical driver and bracing displacement includes every component in the modeled chamber, without double-counting.
Use the manufacturer’s mounting cutout, outer flange, mounting depth and fastener pattern; nominal speaker diameter is not the cutout diameter.
Check driver magnet, brace and port interference, including space at the port openings and the route needed to install the driver.
Confirm the completed outside box dimensions fit the intended vehicle or room installation and that all six panels follow the specified overlap arrangement.

A calculated volume cannot verify airtight joints, adequate cabinet rigidity or actual acoustic response. Check the assembled enclosure and, for a precisely tuned ported box, verify physical tuning after construction.

Model Limitations and Technical Sources

The rectangular-volume and panel-cut relationships are geometric calculations for the stated construction convention. Sealed-driver sizing and straight-port tuning are idealized acoustic estimates: they do not replace manufacturer-specific enclosure guidance, physical fit checks or measured performance.

Cabinet geometry

Uniform wall thickness and six rectangular outer panels are assumed. Wedges, curved walls, double baffles, irregular chambers and alternate joinery require their actual geometry.

Sealed acoustic alignment

The ideal small-signal model does not establish real leakage, absorption, parameter variation or in-car/in-room response. Its system resonance is not interchangeable with measured F3.

Port model

The port solution assumes a straight internal assembly and approximate end corrections. It does not determine physical port orientation, folded-port tuning, airflow noise or the power/excursion limits of a specific driver.

Construction completeness

Use the manufacturer’s actual mounting specifications and physical component layout to validate the build; the outer-panel list is not a complete fabrication drawing.

Technical references

KICKER — How to Build a Subwoofer Box — Manufacturer construction guidance on enclosure dimensions, internal displacement, bracing and ported designs.
KICKER — CompVR Owner’s Manual — Example of model-specific enclosure and physical construction guidance; use the manual for your actual driver rather than assuming its specifications apply universally.

The worked example is independently checked by adding its occupied volume back to its unrounded net volume and converting the resulting gross cubic feet to cubic inches. The acoustic equations describe the simplified models named in their sections, not measured performance.