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Convert heating or cooling load to required airflow. Cooling typically needs 400 CFM per ton (12,000 BTU/hr). Heating airflow is often 50–70% of cooling CFM depending on furnace temperature rise.
CFM = Tons × 400 or CFM = BTU / (1.08 × ΔT)Size round ducts so velocity stays within recommended limits: 700–900 FPM for main trunks, 500–700 FPM for branch runs, and 400–500 FPM for flex connections near registers.
Area (sq ft) = CFM / (Velocity × 60); D = √(4 × Area / π)Convert round duct area to equivalent rectangular dimensions using equal cross-sectional area. Aspect ratios above 4:1 increase friction loss and should be avoided.
W × H = π × (D/2)²Updated: July 2026
A 3-ton system moving 1,200 CFM at 800 FPM requires roughly 270 sq in duct area. A 14-inch round duct (153 sq in) is too small; a 16-inch round duct or 20×10 rectangular trunk is appropriate.
A 6-inch branch at 150 CFM runs at ~760 FPM — within the 500–900 FPM branch range. Drop to 5-inch only if run length is under 15 feet and total external static pressure budget allows.
Return ducts should match supply CFM and use lower velocity (600–700 FPM) to reduce noise. 1,000 CFM at 650 FPM needs ~370 sq in — a 22×8 or 20×10 return grille path works.
Each branch must be sized for its register CFM. A 6-inch branch carrying 200 CFM runs at 1,020 FPM — too fast. Use 7-inch for 200 CFM or reduce damper settings with proper duct sizing.
Each 90° elbow adds 25–30 feet of equivalent straight duct. A run with four elbows behaves like 100+ extra feet, increasing friction loss and requiring upsizing or a higher-static blower.
Undersized ducts restrict airflow, increase noise, and reduce HVAC efficiency. This calculator determines round and rectangular duct dimensions from required CFM, maximum velocity limits, and equivalent length so supply and return runs deliver rated system capacity.