Duct Sizing Calculator
Size round or rectangular ductwork using the equal friction method based on CFM and desired friction rate.
Inputs
CFM
"WC/100ft
Residential: 0.08" WC/100 ft; Commercial: 0.06–0.10"; VAV systems: 0.06–0.08" WC/100 ft
SMACNA best practice: ≤4:1 aspect ratio; >4:1 increases friction loss significantly
Results
Round Duct Diameter
22 inches
Rectangular Width0 inches
Rectangular Height0 inches
Air Velocity303 FPM
Pressure Drop0.08 "WC/100ft
How to Use This Calculator
- Enter Airflow, Friction Rate, and Duct Shape.
- Set Aspect Ratio (W:H).
- Review the Round Duct Diameter (inches) result.
- Use Rectangular Width (inches) and Rectangular Height (inches) to inform your decision.
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