Work out the real net air volume of a sealed or ported subwoofer enclosure from its outside or inside size, wood thickness and everything that sits inside it. For ported boxes it finds the slot or round port length for your target tuning frequency, accounting for the port's own volume, and it gives you a panel cut list ready for the saw.
Last updated:
Enclosure type
The dimensions I enter are
Box size
3/4" = 0.75 in ≈ 1.9 cm MDF is the usual choice
What takes up space inside
From the spec sheet; a 12" sub is often about 0.08–0.15 ft³
Same volume unit; braces, terminal cups, double baffle
Port (ported boxes)
Hz
Low 30s Hz for deep music bass, mid to high 30s for louder, punchier bass
Usually the full inside height of the box
End correction added to the physical length
Driver T/S parameters (optional)
Hz
In the volume unit chosen above
Enter your values to see the result.
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How to use the Subwoofer Box Calculator
Pick sealed or ported and say whether your dimensions are outside or inside measurements.
Enter width, height, depth and wood thickness in inches or centimeters.
Add the driver displacement from the spec sheet, the number of subs and any bracing.
For a ported box, enter the tuning frequency and the slot or round port size to get the port length.
Optionally tick T/S parameters to see the sealed Qtc and F3 or a rule-of-thumb ported size, then download the cut list.
Formulas used
Quantity
Formula
Notes
Inside dimension
outside − 2 × wood thickness
For each of width, height and depth
Gross volume
W × H × D ÷ 1,728 (in → ft³) or ÷ 1,000 (cm → L)
1 ft³ = 28.317 L
Net volume
gross − drivers − bracing − port
The volume the driver actually works against
Effective port length
Lv = c² × Av ÷ (4π² × Fb² × Vb)
c = 34,300 cm/s (air at about 20 °C)
Physical port length
Lv − k × D
k = 0.732 (one flanged end), 0.85 (both flanged), 0.614 (both free); slot ports use the equivalent round diameter
Sealed box Q
Qtc = Qts × √(Vas ÷ Vb + 1)
Fc = Fs × √(Vas ÷ Vb + 1)
Sealed volume for a target Q
Vb = Vas ÷ ((Qtc ÷ Qts)² − 1)
Only possible when Qtc > Qts
Ported rule of thumb (Keele)
Vb ≈ 15 × Vas × Qts2.87, Fb ≈ 0.42 × Fs × Qts−0.9
A starting point, not a full alignment
Slot ports that run along a box wall get a little extra end correction from the walls, so a real slot port often tunes slightly lower than calculated. Build the port a touch long and trim it if you can measure the tuning (the impedance minimum between the two peaks).
Typical net volumes and tuning by sub size
A rough guide only; the driver's spec sheet always wins.
Sub size
Sealed net volume
Ported net volume
Common tuning
Port area at 12–16 in²/ft³
8"
0.3 – 0.6 ft³ (8 – 17 L)
0.5 – 1.0 ft³ (14 – 28 L)
35 – 45 Hz
6 – 16 in²
10"
0.6 – 1.0 ft³ (17 – 28 L)
1.0 – 1.75 ft³ (28 – 50 L)
32 – 40 Hz
12 – 28 in²
12"
1.0 – 1.75 ft³ (28 – 50 L)
1.75 – 3.0 ft³ (50 – 85 L)
30 – 38 Hz
21 – 48 in²
15"
1.75 – 3.0 ft³ (50 – 85 L)
3.0 – 5.0 ft³ (85 – 140 L)
28 – 35 Hz
36 – 80 in²
Building tips
Use 3/4" (18–19 mm) MDF and glue plus screw every joint; seal seams with wood glue or silicone so the box is airtight.
Brace large panels: a dowel or shelf brace between the widest panels stops flexing, but add its volume to the bracing field.
Keep the port opening clear: leave at least one port width (or diameter) between the port's inner end and the nearest wall.
Double baffles are common for heavy subs; add the extra baffle's internal volume to the bracing displacement.
Measure twice: the cut list assumes butt joints with the front and back panels overlapping the top, bottom and sides.
Frequently asked questions
How do I calculate subwoofer box volume?
Multiply the inside width, height and depth. In inches divide by 1,728 to get cubic feet; in centimeters divide by 1,000 to get liters. If you measure the outside, first subtract twice the wood thickness from each dimension. Then subtract the driver displacement, bracing and, for a ported box, the port to get the net volume.
How is port length calculated?
A ported box is a Helmholtz resonator. The effective port length is Lv = c² × Av ÷ (4π² × Fb² × Vb), where c is the speed of sound (about 343 m/s), Av the total port area, Fb the tuning frequency and Vb the net volume. The physical length is shorter by an end correction of about 0.732 × the port diameter for a port with one flanged end.
What tuning frequency should I use for a ported sub box?
Most car and home subwoofer boxes are tuned between about 30 and 40 Hz. Lower tuning (around 28 to 33 Hz) gives deeper, more even bass for music; higher tuning (35 to 40 Hz and up) gives more output and punch higher up but less deep bass. Do not tune far below what the driver's parameters support.
How much port area do I need?
A common rule of thumb for subwoofers is about 12 to 16 square inches of port area per cubic foot of net volume, more for very powerful systems. Too little area causes chuffing (port noise) at high volume. More area needs a longer port for the same tuning, so the tool shows the length so you can check it fits.
Does the port take up box volume?
Yes. The air inside the port and, for a slot port, the extra divider board both reduce the net volume, which in turn changes the tuning. The calculator subtracts them and solves port length and net volume together until they agree.
What is Qtc in a sealed box?
Qtc is the total Q of the driver in the sealed box: Qtc = Qts × √(Vas ÷ Vb + 1). About 0.707 gives the flattest response, values around 0.5 to 0.6 sound tight and extended, and values above about 0.9 sound boomy. A smaller box raises Qtc.
Should I trust the manufacturer's recommended box?
Yes, start there. The manufacturer's recommended net volume and tuning are designed for that driver. Use this tool to hit those numbers exactly with your chosen dimensions, wood and port, and remember that a car cabin adds low-frequency gain that makes the bass sound deeper than the box alone.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.