Learn more: How to Use a Duct Sizing Chart Properly
Duct Sizing Calculator
Calculate the correct duct diameter based on required airflow (CFM) and friction rate.
Tip: Residential systems typically need ~400 CFM per ton of cooling
Inches of water column (″ WC) per 100 feet of duct
Flex duct requires larger sizes due to higher friction
Target: 600-900 FPM for supply, 500-750 FPM for return
About This Calculator
This calculator uses the ASHRAE Equal Friction Method to determine duct sizes. The method maintains a constant pressure drop per unit length throughout the duct system, which simplifies balancing and ensures consistent airflow.
Note: Calculations assume galvanized steel (rigid/sheet metal) duct with standard roughness (ε = 0.0003 ft). For flex duct, select “Flex Duct” above — the calculator automatically adds 1-2 inches to compensate for higher friction losses.
Formula Used
Where:
D = Duct diameter (inches)
Q = Airflow rate (CFM)
ΔP/L = Friction rate (″ WC per 100 ft)
For flex duct, add 1-2 inches to the calculated diameter to compensate for the corrugated interior surface which creates additional friction.
