Sub Box Calculator › Subwoofer port calculator
Subwoofer port calculator
Enter the net box volume, the tuning frequency and the port size. The calculator gives the port length, checks the port area and tells you how much volume the port takes up.
How to use the port calculator
- Net volume — the air volume the sub works with, after the sub, the port and bracing are subtracted. If you only know the outside dimensions, see net vs gross volume.
- Tuning frequency — most car-audio boxes are tuned between 30 and 38 Hz. Lower is deeper but quieter, higher gives more punch.
- Port size — the inside diameter of a round port (or of each port, if there are several), or the inside width and height of a slot.
The result is the physical length of the port: for a round tube, the tube length; for a slot, the length along the middle of the channel. The table below the result shows how the length changes if you tune a few hertz higher or lower.
The port length formula
The port and the air in the box form a Helmholtz resonator. The calculator uses the same formula and end correction as WinISD and our full box calculator:
With several round ports the length is calculated from their total area, and the end correction from one port. A worked example is in how to calculate a ported box.
Round, aero and slot ports
- Round port — a PVC or cardboard tube. Use the inside diameter.
- Aero port — a round port with flared ends. The flares lower air noise; enter the inside diameter of the straight part. Flares change the effective length a little, so treat the result as a starting point and check the tuning of the finished box (see how to tune a sub box).
- Slot port — a rectangular channel built from the same sheet as the box, usually along a side wall. Easy to make large and can be folded inside the box. Its wall also takes up volume, so enter the wall thickness.
How much port area do you need?
A common car-audio rule is 12–16 square inches of port area per cubic foot of net volume (about 2.7–3.6 cm² per liter). Less area makes a shorter port but more air noise at high power; more area is quieter but longer. The rule ignores the sub itself: a long-throw sub moves more air and needs a bigger port. Enter its Sd and Xmax and the calculator checks your port against Richard Small’s minimum vent area, Sp ≥ 0.8 × Fb × Sd × Xmax. The real test is air speed at your amplifier power: under about 66 ft/s (20 m/s) peak the port stays quiet, above about 82 ft/s (25 m/s) chuffing is likely — the full calculator shows it for your sub.
Don’t forget the port volume
The port is inside the box, so it takes up space: add the “port takes up” value to the net volume when you draw the box, otherwise the box will be too small and the tuning will come out high. The full calculator does this automatically, folds a long slot to fit and draws every panel with a cut sheet and DXF.
Model it with your sub→FAQ
How long should a subwoofer port be?
It depends on the box volume, the tuning and the port area. For example, a 2.5 ft³ box tuned to 33 Hz needs a 4″ round port about 9.4″ long, or a 2.5″ × 13″ slot about 27″ long. Bigger port area or lower tuning means a longer port.
What happens if the port is too long or too short?
A longer port tunes the box lower, a shorter port tunes it higher. A port a couple of inches off moves the tuning by a few hertz, so you can fine-tune a round port by trimming it.
How do I calculate a slot port?
Use the inside width and height of the slot; its area is width × height. The formula is the same as for a round port, with the diameter of a circle of the same area in the end correction. Remember to add the slot and its wall to the box volume.
How many square inches of port per cubic foot?
About 12–16 in² per ft³ for most car subwoofers; 10 in² is the practical minimum for moderate power, and SPL builds often use 16–20 in² or more.