Port Length & Tuning Calculator

How long does your port need to be? Give it your net box volume, your target tuning, and the port you're building — slot port or round port. You get the length to cut, the total port area, and where that area sits in the 12–16 in² per ft³ range. Already built it? The reverse check below tells you what your ported box actually tunes to.

Box & Target Tuning

Net internal box volume

Not sure what your net volume actually is? Use the Net Volume Helper first — this needs net airspace, not your box's outside dimensions.

ft³ liters

Target tuning frequency (Hz) Number of ports

Port shape

Round port Slot port (rectangular)

Port diameter

inches cm

Port end condition

One flanged, one free (typical — flush at the box face, open inside) Both ends flanged Both ends free (rare)

Result

Fill in the box volume, target tuning, and port dimensions, then hit Calculate.

Reverse check: I already built a port — what's my real tuning?

Useful if you rounded your cut length to the nearest 1/8" and want to know your real port tuning — the actual resulting Fb.

Net box volume (ft³) Port diameter (in, round port only) Actual physical port length (in)

End condition

Before you cut: what the numbers do and don't tell you

Port area is a floor, not a pass. The 12–16 in² per ft³ rule is a sizing guideline. What actually causes port noise — chuffing — is air speed through the port at the volume you're running. The best-sourced limit is about 17 m/s, 5% of the speed of sound: Small's 1973 figure for a port that is smooth inside with its ends rounded over. A sharp edge chuffs sooner, and rounding or flaring the ends buys headroom; the higher limits often quoted for flared ports (22–25 m/s) have no measurement behind them. This calculator checks your port area against the rule and shows which side of it you're on. It does not compute air velocity — for a high-power build, run the design through a simulator such as WinISD before you cut wood.

Why the end condition matters. The air just outside each end of the port moves with the air inside it, so a port behaves longer than it physically is. The calculator corrects for that, and the correction depends on how each end terminates. The default — flush with the box face outside, hanging free inside — is the honest choice for a typical car-audio enclosure.

A slot port built into a corner tunes lower than the formula says. When the enclosure's own walls form three sides of the port, its ends load more heavily and it behaves longer still. This tool models a free-standing port, so for a wall-shared slot treat the tuning shown as an upper bound and confirm the real number with an impedance sweep once it's built.

Tuning drifts with the air temperature. This calculator uses the speed of sound at about 71°F (21.6°C). A built port follows the air it sits in: a box tuned to 33 Hz here measures about 31 Hz at 14°F and about 35 Hz at 140°F, roughly 0.1 percent per degree F. Sweep it at the temperature you actually listen at.

Keep the inner end of the port clear. A round port needs at least one port diameter of open space between its inner end and the facing wall; a slot port needs at least its smallest dimension. Closer than that and the port detunes the box after it's built — which is the expensive time to find out.

Every formula here was checked against independently published worked examples before the tool went live. The math runs in metric and converts for display, because several "imperial" versions of the port-length formula circulating online use constants that don't agree with each other — if you pulled one from a forum, re-derive it before trusting it.