Port Length Calculator — Bass Reflex Speaker Enclosure
Enter the port diameter, internal box volume and desired tuning frequency to get the exact port tube length that tunes your bass-reflex enclosure to the target frequency.
cm
litres
Hz
Cut each port tube to this length (outside the box)
- 1
Total port area
1 × π × 2.5² = 19.635 - 2
Angular frequency (ω)
2π × 40 = 251.33 - 3
Effective port length
34,300² × 19.635 ÷ (251.33² × 20,000) = 18.286Helmholtz formula rearranged to solve for port length. - 4
End correction
0.732 × 5 = 3.66 - 5
Port length (cm)
18.286 − 3.66 = 14.6
How does this calculator work?
Port length (cm) = c² × π(d/2)² / ((2π × fb)² × Vb_cm³) − 0.732 × d, where c = 34 300 cm/s. For a 20-litre box, 5 cm port, tuned to 40 Hz: ≈ 14–15 cm (5.7 in). Lower tuning, smaller diameter, or larger box all lengthen the port. Multiple identical ports shorten each individual tube proportionally.
Formula
How this is calculated
A bass-reflex enclosure is a Helmholtz resonator — a rigid cavity connected to the outside by a tube (the port). At the tuning frequency f_b the port and the internal air mass resonate in phase, reinforcing bass output and unloading the woofer cone. Above f_b the port rolls off and the system behaves like a sealed box; below it, output falls steeply and cone excursion rises sharply.
The Helmholtz formula f_b = (c / 2π) × √(A / (V_b × L_eff)) is rearranged to solve for effective port length: L_eff = c² × A / ((2π × f_b)² × V_b). The end correction 0.732 × d subtracts the acoustic length added at the open end of the port (one flanged, one open) to give the physical cut length. Use c = 343 m/s (34 300 cm/s) at 20 °C; temperature changes shift f_b slightly (roughly 0.17%/°C).
Multiple ports of identical diameter are supported — their combined radiating area equals n_p × π(d/2)², which shortens each individual tube length. Choosing the right port diameter is a trade-off: too narrow and the air velocity through the port causes chuffing noise at high output; a common guideline is port area ≥ (Sd × X_max) / (10 × port velocity threshold), or simply use 5–8 cm (2–3 in) diameter for most 10–12 in woofers and subwoofers. This calculator does not account for port flare, non-circular shapes, or slot ports.
Frequently asked questions
A common starting point is 70–90% of the driver's free-air resonance (Fs). Lower tuning gives deeper bass extension but demands a larger box or longer port and increases cone excursion below f_b. Match f_b to your driver's Thiele–Small parameters and target application (music vs cinema vs car audio).
A larger port cross-section A allows the same tuning frequency with a shorter effective length, because the Helmholtz equation shows L_eff is proportional to A. However, a wider port also has higher air velocity at a given volume velocity, potentially causing turbulence and port noise at loud levels.
A negative result means the end-correction term (0.732 × d) exceeds the theoretical effective length — the port would need to be shorter than its own end correction. This usually means the port is too wide for the box or the tuning frequency is too high. Try a narrower diameter, lower the tuning frequency, or increase the box volume.
Also known as
TG we-Calculate Editorial Team. (2026). Port Length Calculator — Bass Reflex Speaker Enclosure [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/port-length-calculator
TG we-Calculate Editorial Team. "Port Length Calculator — Bass Reflex Speaker Enclosure." TG we-Calculate. 2026. https://we-calculate.com/calculator/port-length-calculator.
TG we-Calculate Editorial Team, "Port Length Calculator — Bass Reflex Speaker Enclosure," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/port-length-calculator
@misc{wecalculate_port_length_calculator, title = {Port Length Calculator — Bass Reflex Speaker Enclosure}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/port-length-calculator}}, year = {2026}, note = {TG we-Calculate} }
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