Formula
D = (0.109136 · Q^1.9 / ΔP)^(1/5.02) · Vel (FPM) = CFM ÷ (πD²/4)Sizes round galvanized duct from CFM and target friction rate, then shows the equivalent rectangular sizes for common widths. Verify against ACCA Manual D for a full design.
Round diameter
11in (exact 10.27")
Nearest commercial size (round up)
Velocity
606FPM
At 11" round · target 700–900 FPM
Residential supply trunk 700–900 FPM (branches 600–700). Duct may be oversized — verify branch balancing.
Equivalent rectangular (in × in)
6 × 16
8 × 12
10 × 9
12 × 8
14 × 7
16 × 6
18 × 6
20 × 5
22 × 5
24 × 5