FREE HVAC TOOL
Duct Sizing Calculator
Estimate a preliminary duct size from airflow and target air velocity.
Estimated duct size
—
Enter airflow and review the estimate.
Required area—
Velocity at selected nominal size—
Round equivalent—
How to Use
- Enter the airflow in CFM.
- Select duct type and application. The velocity field starts with an editable estimate.
- Choose round or rectangular duct. For rectangular duct, enter the width.
- Review the estimated nominal size and calculated velocity.
Assumptions and Limitations
This is a preliminary sizing estimate using airflow divided by target velocity. It does not calculate friction rate, total effective length, fitting losses, system static pressure, blower performance, balancing, or acoustic performance. Flex duct resistance depends on installation quality, including sagging and compression. Verify final duct design using the applicable HVAC design method and equipment data.
Velocity presets are editable starting points, not universal code requirements or a substitute for professional design.
