About the Duct Size Calculator
Round or rectangular duct size for a required airflow at a sensible velocity.
Also known as: ductwork sizing, air duct diameter, hvac duct calculator, duct velocity.
How to use this calculator
- Enter airflow, target velocity, duct shape.
- The result updates instantly as you type — press Calculate (or Enter) at any time.
- Review the breakdown shown under the main result.
- Use Copy result or Share to save the answer or send a link with your inputs.
Formula
A = Q / v · d = √(4A / π)
Worked example
Using the values pre-filled in the calculator above:
- Airflow = 400 CFM
- Target velocity = 5 m/s
- Duct shape: Round
the calculator returns:
- Duct diameter: 250 mm
- Exact diameter needed: 219.3 mm
- Velocity in that duct: 3.846 m/s
- Required area: 37,756 mm²