Online Tool Store
· 4 min read
How to Size Ventilation Duct for an Airflow
Heshan Fernando
Co-founder & COO
An extract fan rated at 360 m³/h connected to 100 mm flexible duct will not move 360 m³/h. It will move considerably less, make a noise doing it, and the homeowner will conclude the fan is faulty.
Duct sizing is where ventilation systems quietly fail, and the reasoning behind it is simpler than the pipework suggests.
Velocity is the design variable
The relationship is straightforward:
area (m²) = flow (m³/s) ÷ velocity (m/s)
360 m³/h is 0.1 m³/s. At a residential target velocity of 3 m/s, that needs 0.033 m² of cross-section — about 205 mm diameter round.
The interesting question is why 3 m/s rather than 6. Push the same flow through half the area and the velocity doubles. The air still moves, but two things happen: the duct gets audibly noisy, and the pressure drop rises steeply — roughly with the square of velocity — which means the fan works harder and delivers less.
That’s why residential supply duct is sized around 3 m/s while commercial main runs might use 6 to 8. In an office plant room nobody hears it. In a bedroom, they do.
Round versus rectangular
Round duct is more efficient for the same cross-sectional area, easier to seal, and generates less turbulence. Rectangular exists because ceiling voids have depth limits.
The catch is that a rectangular duct’s equivalent diameter — the round duct that performs the same — is always smaller than its dimensions suggest, and the more extreme the aspect ratio, the worse it gets. A 400 × 100 duct performs considerably worse than its area implies. Keep the aspect ratio below about 3:1 where you can.
| Application | Target Velocity | Reason |
|---|---|---|
| Residential supply | ~3 m/s | Quiet |
| Residential extract | 3–4 m/s | Slightly more tolerant |
| Commercial main | 6–8 m/s | Cost of duct vs. noise |
Why people get stuck here
- Sizing from the fan’s spigot. The fan outlet is a connection size, not a duct size.
- Flexible duct. Its resistance is far higher than smooth rigid duct, and a compressed or kinked run can halve the flow.
- Fittings ignored. A tight bend can cost more pressure than several metres of straight duct.
- Velocity as the only calculation. Sizing for velocity is the start; total system pressure determines whether the fan can actually deliver.
What good duct sizing looks like
Sized from flow and velocity, then rounded up
Take the calculated area, pick the next standard size up, never down.
Fittings counted
Every bend, reducer and grille adds resistance. A run with six tight bends behaves like a much longer run.
Flexible duct minimised
Use it for short, straight connections only, pulled taut. A metre of compressed flexible duct can undo the benefit of correctly sizing everything else.
The full pressure drop checked against the fan curve
Velocity sizing tells you the duct is quiet enough. The fan curve tells you whether it will actually move the air.
Common mistakes to avoid
- Connecting 100 mm duct to a fan because the spigot is 100 mm.
- Running flexible duct in long, sagging loops through a loft.
- Using a high aspect ratio rectangular duct where a round one would fit.
- Forgetting the terminal grille, which has its own significant pressure drop.
- Sizing for velocity and never checking the total system resistance.
How to do it with Duct Size Calculator
The Duct Size Calculator sizes for a target velocity and gives round and rectangular equivalents.
- Enter the airflow the system must move.
- Choose the application, which sets the acceptable velocity.
- Read the required area, diameter, and rectangular equivalent.
- Pick the next standard size up, then check the full run’s pressure drop against the fan’s curve.
CIBSE publishes the design guidance used for commercial systems if you need authoritative figures. Other building services calculators are in the tools directory.
Frequently asked questions
Why does duct velocity matter?
Because it drives noise and pressure drop. Residential supply duct sized near 3 m/s is quiet; the same flow squeezed through half the area is audible in every room it serves.
Round or rectangular?
Round is more efficient and easier to seal. Rectangular exists for depth constraints, and its equivalent diameter is always smaller than the dimensions suggest — keep the aspect ratio moderate.
Does duct length matter?
Yes. Velocity sizing sets the diameter; total pressure drop across the run and its fittings determines whether the fan can deliver the flow. Bends cost far more than straight length.
Final thought
Size for velocity first, then check the fan curve against the whole run. A correctly sized duct on an undersized fan moves no more air than an undersized duct does.