Duct Size Calculator
Find the round duct diameter for a given airflow. Enter the CFM and a design air velocity to see the required cross-section and the equivalent round duct size.
How to size a round duct
- Enter the airflow through the duct in CFM.
- Choose a design air velocity in feet per minute.
- Read the required duct area in square inches.
- Use the equivalent round diameter to pick a duct.
Examples
400 CFM at 700 fpm
400 CFM, 700 feet per minute
About 82.29 sq in, roughly a 10 in round duct
1000 CFM at 900 fpm
1000 CFM, 900 feet per minute
160 sq in, roughly a 14 in round duct
Frequently asked questions
How is duct size calculated from CFM?
Divide the airflow in CFM by the design air velocity in feet per minute to get the cross-section in square feet, then convert to a round diameter.
What air velocity should I use?
Around 700 to 900 feet per minute is common for residential supply ducts, with lower speeds for quieter return air. These are guidelines, not a substitute for a Manual D design.
Why convert to a round diameter?
Flexible and rigid round ducts are sold by diameter. The tool turns the required area into the equivalent round size so you can buy the right duct.
What about rectangular ducts?
Pick a rectangular duct with the same cross-sectional area, or use an equivalent-diameter chart. This tool reports the round size, which is the usual reference.
Does a higher velocity mean a smaller duct?
Yes. Moving the same air faster needs less cross-section, so the duct is smaller, but higher velocity can add noise and friction loss.
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