Duct sizing calculator
Enter an airflow and a target velocity or friction rate, and get a round duct and a rectangular duct both sized to the same target — using the same Darcy-Weisbach and Colebrook-White engine as the Duct Network Configurator, not a textbook approximation. See both shapes drawn overlaid, to scale.
How this calculator works
Given an airflow and a target velocity, the required cross-section area follows
directly: A = Q / v. Given a target friction rate instead, there's no
closed-form solution — the calculator bisects on duct size, running the same
Darcy-Weisbach / Colebrook-White friction calculation as the
Duct Network Configurator at each trial
size until the resulting friction rate matches your target.
The round and rectangular ducts are sized independently against the same target — not converted from one to the other via the traditional Huebscher equal-friction chart formula. That chart is itself only an approximation; solving both shapes directly against the same physics gives an exact match instead.
Velocity target: A = Q / v, then D = √(4A/π) (round) or W×H = A (rectangular)
Friction target: bisect duct size until Δp/L = f · (L/Dh) · (ρv²/2) / L matches target
Dh = 4A/P (hydraulic diameter, rectangular) f = Colebrook-White
Both results are then rounded up to a standard size — the round duct to the nearest size in the common EU/ISO spiral duct series, the rectangular duct to the nearest 25 mm on each side — so the numbers you get are ones you can actually order.
Worth not mixing up: the hydraulic diameter shown for the rectangular duct is just the geometry term fed into the Reynolds number and friction factor above — it is not an "equivalent round duct size". The actual round-duct equivalent is the Ø value in the Round column, sized independently against the same target.
Acoustic check
Higher velocity means more self-generated noise at fittings, dampers and diffusers — but that relationship depends on the specific fitting, and isn't something this page tries to turn into an absolute sound level. Instead, the optional acoustic check compares your sized velocity against the recommended and maximum air velocity tables by required noise level, from Venture Industries Group's ventilation & air conditioning design guide (§8.1, "Zalecane prędkości powietrza").
This page sizes one duct run at a time. CloudAir's selection programs run the same engineering calculations across a whole project and generate the schedule automatically. See how CloudAir handles it →