Round duct capacity by friction rate
Rectangular duct capacity by friction rate
One duct size across airflows
Job examples
- Will a 12" carry it?Check a 12" round for 600 CFM.Capacity table: 605 CFM at .08, about 768 FPM.
- Check an existing rect trunk24 × 12 trunk carrying 1600 CFM.800 FPM at .062 friction, fine for a main trunk.
- Rect branch for 600 CFM8" deep is all the joist space you have.Rect capacity table, 8" deep: 16 × 8 carries 630 CFM at .08.
How it's calculated
Same friction equation as the Duct Sizer, standard air, galvanized duct, per 100 ft (per metre in metric). Capacity is the airflow that gives that friction rate in each size. Rectangular duct uses its ASHRAE equal-friction round size for the friction, and its real area (width × depth) for the velocity, so a rectangle runs slower than its round equivalent at the same airflow.