Slot Port Calculator: Length, Area and Displacement
Slot port length for your tuning, its area per ft³ and how much box it takes up, wall included.
Find
Port
length
Tuning
(Hz)
Port length
Tuning (Hz)
Port length
Tuning (Hz)
Slot width, the gap (in)
Slot height (in)
Usually the inside height of the box.
Port wall thickness
= 19 mm
The board that forms the slot.
Net box volume (ft³)
Net: after sub, port and bracing displacement.
Tuning (Hz)
30–36 Hz is usual for a daily. Lower = deeper.
Slot length
37 3/8" (950 mm)
Port area
40.5 sq in, 16.2 sq in per ft³ (typical 12–16)
Port takes up
1.1 ft³ (31 L)
Box volume to build (net + port)
3.6 ft³ (101.8 L)
Formula
L = 1.463 × 10⁷ × N × R² ÷ (Fb² × Vb) − k × D
With
Fb = 32 Hz, Vb = 4,320 in³, R = 3.59", N = 1, k × D = 5.25", L = 37 3/8"
Starting points, not guarantees. Your sub’s manual wins: check its recommended box and tuning.
How it works
Enter the slot width (the gap), its height (usually the inside height of the box), the wall thickness, the net volume and the tuning. You get the slot length, its area per ft³, and how much box it takes up, the dividing wall included.
Example: a 3 × 13.5 in slot (40.5 sq in) in a 2.5 ft³ box tuned to 32 Hz is 37.38 in long. That is 16.2 sq in per ft³, at the top of the usual range.
With a 3/4 in wall it takes up (40.5 + 0.75 × 13.5) × 37.38 in³ = 1,892.5 in³, or 1.1 ft³: build the box 3.6 ft³ gross for 2.5 ft³ net, before the sub’s displacement.
Frequently asked questions
How do I calculate a slot port?
Treat it as a round port with the same area: radius = √(area ÷ π). Then use the round-port formula. A 12.5 × 7.25 in slot (90.6 sq in) in a 6.375 ft³ box tuned to 35 Hz comes out about 23.4 in long.
How much port area do I need?
About 12 to 16 sq in per ft³ of net volume is a common range for a daily box. Less can chuff at high volume; more makes the port very long.
Does the port wall count against the box volume?
Yes. Both the air inside the slot and the board that forms it take up room. Add them to the net volume to get the size you build.