Online Tool Store
Duct Size Calculator
Size round or rectangular duct for an airflow target using velocity limits, and see the equivalent diameter and expected pressure drop.
🔒 This tool runs entirely in your browser. Your files are never uploaded to a server.
Unit & Converters
Duct Size Calculator
Frontend preview — no upload or external service.
Duct size
360 m³/h at a 3 m/s residential velocity needs about 0.033 m² — a 205 mm round duct, or 200 x 170 mm rectangular.
How the Duct Size Calculator works
- Enter the airflow the system must move, in m³/h or l/s.
- Choose the application, which sets the acceptable velocity — residential duct is sized for quiet, commercial for cost.
- Read the required area and pick the next standard size up, never down.
The method
Duct area is airflow divided by the target velocity, and the diameter follows from the area of a circle.
area (m²) = flow (m³/s) / velocity (m/s); diameter = √(4 x area / π)
360 m³/h is 0.1 m³/s; at 3 m/s that needs 0.033 m², giving a diameter of about 0.205 m.
FAQ
Why does duct velocity matter?
Because velocity generates noise and pressure drop. Residential supply duct is usually kept near 3 m/s for quiet; pushing 6 m/s through the same duct is audible in every room it serves.
Round or rectangular?
Round is more efficient for the same area and easier to seal. Rectangular exists for depth constraints, and its equivalent diameter is always smaller than its dimensions suggest.
Does duct length matter?
Yes. This sizes for velocity; total pressure drop across the run and its fittings determines whether the fan can actually deliver the flow. Bends cost far more than straight length.
How we compare
| Feature | Online Tool Store | A search-engine answer box | A reference table |
|---|---|---|---|
| Velocity-based sizing | ✓ | Rule of thumb | Yes |
| Round and rectangular equivalents | ✓ | ✗ | ✓ |
| Explains the noise trade-off | ✓ | ✗ | Sometimes |
| Full system pressure calculation | ✗ | ✗ | ✓ |
Duct Size Calculator sizes for velocity, which sets noise — the full pressure-drop calculation across the run still decides whether the fan can deliver it.