Job examples
- Swap a rect run for roundExisting 12 × 8 trunk, want to go round.10 11/16" equivalent, next size up 12".
- Oval in a wall cavity20 × 8 flat oval, what round does it match?13 3/16" equivalent, next size up 14".
- Get under a beam16" round has to fit through a 6" space.44 3/8 × 6 rectangular (7.4 : 1).
How it's calculated
ASHRAE equivalent diameter (equal friction at equal airflow). Rectangular: De = 1.30 (a·b)0.625 ÷ (a+b)0.25. Flat oval: De = 1.55 A0.625 ÷ P0.25, with A = πb²/4 + b(a−b) and P = πb + 2(a−b). Fit a depth solves the exact width (or oval major axis) that gives the same equivalent round at a fixed depth.