Round duct air velocity Calculator: measuring notes and method
Use airflow and clear internal area
Enter total volume flow through that duct section and the actual internal diameter. A larger diameter at the same flow lowers the average velocity because the cross-sectional area increases.
Average velocity has limits
Local readings can differ across the duct. Compare a measured reading with the cross-sectional average only when the measurement method supports that comparison.
Formula
Velocity (m/s) = airflow (m³/h) ÷ 3600 ÷ [π × (diameter ÷ 2)²].
Worked examples in metric and imperial units
Illustrative values match Reset example and stay fixed when you edit the calculator. Both columns describe the same calculation; converted inputs are rounded.
Metric worked example
- Airflow
- 500 m³/h
- Internal duct diameter
- 300 mm
Average air velocity: 1.9649 m/s
Imperial worked example
- Airflow
- 294.2889 CFM
- Internal duct diameter
- 11.811 in
Average air velocity: 6.4464 ft/s
Follow the calculation
Lengths in this working are in metres.
- Flow = 500 ÷ 3,600 = 0.138889 m³/s.
- Area = π × (0.3 ÷ 2)² = 0.070686 m².
- Average velocity = volume flow in m³/s ÷ area in m².
Reviewed 19 October 2026. How calculations are checked · Report a correction