HVAC Duct Sizing Calculator: CFM to Duct Size (Round + Rectangular)
HVAC Duct Sizing Calculator
Enter CFM and friction rate — get round duct diameter and rectangular equivalents to Manual D.
Quick answer
Manual D duct sizing: pick a friction rate (0.1 in. w.c./100 ft), respect velocity limits (≤900 FPM supply), and add fitting equivalent lengths. Calculator and method below.
Enter airflow (CFM) and friction rate to size round or rectangular ducts. Based on the Darcy-Weisbach approximation used in ACCA Manual D.
Reference: HVAC Duct Sizing Chart (CFM to Duct Size). Always verify against ACCA Manual D and local code.
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