Round bass-reflex port

The calculation

S = N × π × (D / 2)²
Leff = c² × S / (4π² × Fb² × Vb)
L = Leff − k × (D / 2)
c = 343 m/s; SI units inside the formula

Assumptions & limits

Helmholtz approximation for straight round ports. Net volume excludes the port assembly and other solid displacement. Flared ends, nearby walls, losses and air velocity need separate assessment. k is the selected total end correction in radii.