Airflow and ducts
Circular duct area and air velocity
- where
- cross-sectional area (m²)
- internal duct diameter (mm)
- mean air velocity (m/s)
- air volume flow rate (m³/s)
Explain this equation
Works out the internal area of a round duct and the average air speed through it. Use it to check whether a duct size will be quiet and have an acceptable pressure drop.
D is the internal diameter in millimetres. π D²/4 is the area of a circle, and dividing by 1 000 000 turns mm² into m². V̇ is the airflow in m³/s; dividing it by the area gives the mean velocity v in metres per second.
Flow equals area times velocity, so for a fixed flow the velocity is inversely proportional to area, which goes with the square of the diameter.
Sense check: 0.2 m³/s through a 200 mm duct gives about 6.4 m/s. Doubling the diameter quarters the velocity.
Watch out: use the true internal size; lined ducts and flexible ducts have a smaller clear bore than their nominal size.
Air velocity in a round duct from the flow and the diameter. Use it to check a duct against noise and pressure-drop limits.
Enter your values to see the result.
Good to know
The 4 000 000 converts mm² to m² (π D²/4 with D in mm, divided by 10⁶). Velocity pressure is 0.6 v² for standard air. Low-pressure commercial systems commonly run mains at around 4–7 m/s and branches lower near occupied spaces; CIBSE Guide B gives recommended velocities by application.
Common mistake
Using the nominal outside diameter of a flexible duct or a duct with internal lining as if it were the clear internal size.
Limits
Mean velocity only; the profile across the duct is not uniform.
Sources
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow the equipment manufacturer’s data, CIBSE guidance, BS EN 378, the F-gas Regulations and your company procedures. Refrigerant work needs the correct F-gas certification.