Duct Sizing Methods
Duct sizing uses the equal friction method, which determines duct size based on a uniform friction rate throughout the system. The round duct diameter is calculated using:
D = 0.109 * (CFM^0.5) * (friction_rate^-0.205)
Rectangular equivalent: De = 1.30 * ((a*b)^0.625) / ((a+b)^0.25)
- Residential: Friction rate of 0.08 in. w.g./100ft is typical
- Commercial: 0.06-0.10 in. w.g./100ft is common
- Flex duct has about 10x the friction of sheet metal
- Keep aspect ratio below 4:1 for rectangular ducts