Job examples
- Bathroom fan run100 CFM fan, sized at .10 friction.5 13/16" exact, so run 6" round.
- Basement suite supply trunk1000 CFM off the furnace at .08.16" round, or 24 × 10 / 20 × 12 rectangular.
- Commercial TI, tight ceiling3500 CFM at .05, only 10" of depth above the T-bar.About 70 × 10. That is past 4 : 1, so confirm with the engineer.
How it's calculated
Standard air, galvanized round duct. Friction: f = 0.109136 × Q1.9 ÷ D5.02 (in. w.c. per 100 ft, Q in CFM, D in inches), the same duct friction chart used by common duct slide calculators. Velocity = CFM ÷ area. Velocity pressure = (V ÷ 4005)². Rectangular and oval sizes have the same friction as the round size (ASHRAE equivalent diameter), widths rounded up to the next even inch, up to the aspect ratio you pick. Fit a depth solves the exact width (or oval major axis) for a fixed depth.