Free tool · HVAC technical
CFM & duct size calculator
Airflow from tonnage, and the round duct size for a target friction rate.
System airflow
1,200 CFM
3 tons × 400 CFM/ton
Round duct diameter
10.3″ → use 11″
Equal-friction equation, galvanized round duct
Velocity
695 FPM
Within a typical residential range.
Friction loss ≈ 0.109136 × CFM^1.9 ÷ D^5.02 (D in inches). Flex duct has higher friction: size up. Use ACCA Manual D for full designs.
How it works
Airflow first: most systems are designed around 350–450 CFM per ton of cooling, depending on equipment and climate. Then each duct run is sized to carry its share of that air at the friction rate your design allows.
The duct size uses the standard equal-friction equation for round galvanized duct, the same relationship behind the duct calculators and friction charts techs carry.
The formula
System CFM = tons × CFM per ton. Friction loss per 100 ft ≈ 0.109136 × CFM^1.9 ÷ D^5.02, solved for diameter D in inches. Velocity (FPM) = CFM ÷ duct area in square feet.
Frequently asked questions
400 CFM per ton is the common rule, but the right figure depends on the equipment and climate (often 350 in humid climates for better moisture removal). Use the manufacturer's data.
Sources
Last reviewed October 15, 2026. Third-party sources are marked; some are written by competing vendors.
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