Duct Size Calculator
Duct diameter for a target airflow.
Enter your values
ⓘ Live results
Results
- Round duct diameter
- 11.1 in
- Duct area
- 96 in²
- Rectangular width
- 12 in
- Next standard size
- 12 in
Quick answer
Duct size follows from airflow divided by the target air velocity.
Area = CFM ÷ velocity
How to use the Duct Size Calculator
- 1Enter the airflow the branch must carry.
- 2Choose a target velocity for noise control.
- 3Enter duct height for a rectangular equivalent.
Worked example
Using the values the calculator loads with:
Inputs
- Airflow600 CFM
- Target velocity900 ft/min
- Rectangular duct height8 in
Results
- Round duct diameter11.1 in
- Duct area96 in²
- Rectangular width12 in
- Next standard size12 in
Frequently asked questions
What velocity is quiet?
700–900 ft/min for residential supply ducts.
Why not use smaller ducts?
Velocity noise and static pressure rise sharply.
About this calculator
- Built and reviewed by James Whitfield · How we build and test calculators
- Report an error
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.