Duct design
Duct Size Calculator
Estimate a round duct diameter from CFM and target velocity.
Calculator
How to use
Enter the airflow and a project-selected target velocity. The result is a theoretical round diameter before selecting an available duct size.
Formula
Area = CFM ÷ FPM; D = 12 × √(4 × Area ÷ π)
Variables
CFM is cubic feet per minute, FPM is feet per minute, and D is duct diameter in inches.
Example
At 800 CFM and 700 FPM, the required area is about 1.14 ft² and the calculated diameter is about 14.5 in.
Result interpretation
Choose a practical duct size and recheck velocity, friction loss, fittings, noise, and local requirements.
FAQ
Is a lower velocity always better?
No. Lower velocity may require larger ductwork; acceptable velocity depends on the system, noise target, and design method.