Work out the airflow through a duct from its size and the air velocity — the flow-area-velocity relationship at the heart of duct design, in L/s and m³/h.
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Start free trialA round duct’s area is π × (diameter ÷ 2)²; a rectangular duct’s area is width × height. Work in metres for the flow to come out right.
Airflow equals the duct’s cross-sectional area times the air velocity — bigger duct or faster air both mean more flow.
The result in cubic metres per second is scaled to the L/s and m³/h that HVAC work is usually specified in.
Airflow equals cross-sectional area times velocity: Q = A × V. A 300 mm round duct has an area of about 0.071 m²; at 4 m/s that carries roughly 283 L/s (about 1,018 m³/h). Keep the area in square metres and velocity in metres per second for consistent results.
It’s a balance. Low velocities (around 3–5 m/s in branch duct) keep noise and pressure loss down but need bigger ducts; higher velocities save space but increase noise and fan energy. The right figure depends on the application and where the duct runs.
Too high and ducts get noisy and waste fan energy through friction; too low and ducts become oversized and costly, and may not carry contaminants in extract systems. Selecting a sensible velocity is central to good duct design.
It gives the flow-area-velocity relationship, which is the core of duct sizing. A full design also considers pressure loss, noise, balancing and the system layout — see our duct size and static pressure calculators.
This calculator is a general engineering aid based on the flow = area × velocity relationship. Duct design must also address pressure loss, noise, balancing and standards such as AS 1668, and should be carried out by a competent HVAC designer.
traqR keeps mechanical jobs, duct schedules and commissioning records against each site. Try it free for 14 days.