Duct Size Calculator
Velocity is what makes ductwork audible.
Work out Duct Size. Velocity is what makes ductwork audible. Names the misconception directly.
Round duct diameter
210 mm
or 264 × 132 mm rectangular · 4 m/s
Duct area is simply airflow divided by velocity, so the whole question is what velocity to design for. Residential systems aim at 3 to 5 m/s in branches; commercial systems tolerate more because they are further from occupied space and better attenuated. Velocity is the variable that matters and the one that gets ignored, because a smaller duct is cheaper, easier to route and quieter to specify than to live with. Doubling the velocity quarters the duct area and roughly quadruples the friction loss. The friction figure covers straight duct only. Bends, transitions, grilles and filters typically add as much again — often considerably more in a domestic system with a tight route — so treat it as a floor rather than an estimate.
How the Duct Size Calculator works
Duct diameter for an airflow at a target velocity, with a rectangular equivalent and the friction loss. Above about 5 m/s residential ductwork becomes audible, and the noise rises steeply from there.
Also known as: what size duct for this airflow · why is my ventilation noisy · duct velocity limits · round to rectangular duct
Frequently asked questions
How do I size a duct?
Divide the airflow by the target velocity to get the required cross-sectional area, then convert to a diameter. The whole question is what velocity to design for.
What air velocity should I use?
Three to five metres per second in residential branches. Commercial systems tolerate more because they are further from occupied space and better attenuated, but domestic ducting at 8 m/s will be heard.
Why does undersized ductwork cause noise?
Because noise rises steeply with velocity — roughly as its fifth or sixth power. Halving a duct's area doubles the velocity and makes it dramatically louder, which is why undersizing is the commonest complaint about ventilation systems.
Do bends matter?
A great deal. The friction figure here covers straight duct only; bends, transitions, grilles and filters typically add as much again, and often much more in a tight domestic route.
Round or rectangular duct?
Round is more efficient — less surface area per unit of flow, so less friction — and easier to seal. Rectangular fits into ceiling voids and joist spaces, which is usually why it gets used.
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