Duct Calculator — Duct Size, Velocity & Static Pressure for Fan Selection
The number most fan buyers skip. Enter the airflow, duct size, length, fittings and any filters or grilles; the calculator returns the system static pressure (Pa and in. w.g.), the air velocity with a noise check, and a duct size that keeps velocity sensible. Use the result to read your fan curve at the right point.
No signup · Works on mobile · Formulas shown below · Updated 2026-08-23
Static Pressure of a Duct Run
Total static = duct friction + fittings + components. Darcy–Weisbach with Swamee–Jain friction factor.
Results
Engineering estimate for preliminary sizing. Verify against the fan curve, local codes and your system measurements before purchase. Ask a Longwell engineer to check your duty point free of charge.
How static pressure is calculated
A fan only "sees" resistance: the sum of friction along the duct, the turbulence losses in every fitting, and the pressure drop across components such as filters, grilles and hoods. That total is the static pressure the fan must develop at your airflow — the point on the fan curve you must select against.
Duct friction Δpf = f × (L ÷ Dh) × ½ ρ V² // Darcy–Weisbach Friction factor f = 0.25 ÷ [ log₁₀( ε/(3.7 Dh) + 5.74/Re0.9 ) ]² // Swamee–Jain (turbulent) Fittings Δpk = Σ K × ½ ρ V² Rectangular duct Dh = 1.30 × (a·b)0.625 ÷ (a+b)0.25// equivalent round diameter for equal friction Total ps = ( Δpf + Δpk + components ) × safety factor
Velocity pressure ½ρV² is the hidden driver — it doubles when velocity rises 41 %, which is why the quickest fix for a system that "has no airflow" is a bigger duct, not a bigger fan.
Given: 600 m³/h through 150 mm flexible duct, 5 m long, two bends, carbon filter (120 Pa), 15 % safety.
Area 0.0177 m², velocity 9.4 m/s, velocity pressure 53 Pa
Flex duct ε = 3 mm → f ≈ 0.049 → friction ≈ 87 Pa for 5 m
Bends 2 × 0.9 × 53 = 95 Pa, entry/exit ≈ 53 Pa
Total ≈ (87 + 148 + 120) × 1.15 ≈ 410 Pa — a "600 m³/h" axial in-line fan will deliver perhaps 250 m³/h here. Use a 150 mm EC mixed-flow or backward-curved duct fan, or step up to 200 mm duct (velocity 5.3 m/s → total ≈ 200 Pa).
Frequently Asked Questions
What is a typical static pressure for a duct system?
Residential exhaust fans: 25–125 Pa (0.1–0.5 in. w.g.). Residential HVAC: 125–250 Pa external. Commercial AHU systems: 250–750 Pa. Kitchen hoods and dust collection: 500–1,500 Pa. If you do not know, measure it or calculate it here — guessing is the most common cause of under-performing fans.
How do I calculate duct friction loss?
Use Darcy–Weisbach: friction = f × (length ÷ hydraulic diameter) × ½ρV². For galvanised duct at 5 m/s this is about 1 Pa per metre in a 250 mm duct, rising to 3 Pa/m in a 100 mm duct; flexible duct is 5–10 times worse.
How much pressure drop does an elbow add?
An elbow adds K × velocity pressure. At 5 m/s velocity pressure is 15 Pa, so a smooth elbow (K 0.3) costs ~5 Pa and a mitred elbow (K 1.2) ~18 Pa. In a flex-duct system at 9 m/s each bend costs 40–50 Pa.
What duct size do I need for a given CFM?
Pick a velocity first (1,000 fpm / 5 m/s residential-commercial), then area = airflow ÷ velocity. 400 CFM at 1,000 fpm needs 0.4 ft² → an 8–9 in duct. The calculator suggests the next standard round size.
Why is my new fan not moving more air than the old one?
Because the system, not the fan, sets the airflow. If the ducting is the bottleneck, a larger fan just moves the operating point up the same steep system curve. Cut the static pressure (bigger or shorter duct, fewer bends, clean filters) before buying a bigger fan.
Link to or Cite This Tool
This calculator is free to reference in articles, forums, specifications and course material. Please credit it with a link:
<a href="https://www.longwellfans.com/resources/duct-static-pressure-calculator/">Duct Size & Static Pressure Calculator</a> by LONGWELL Fans
