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Duct size for CFM
Smallest round-duct diameter that carries your target CFM under a chosen friction-loss rate. Low-velocity commercial default is 0.08 in WC per 100 ft of straight duct.
Round duct
20″ diameter
Exact: 18.89″; rounded up to next 2″
Equivalent rectangular (2:1)
26″ × 13″
ASHRAE Deq formula at the rounded diameter
Common friction targets: 0.08 in WC / 100 ft (low-velocity commercial), 0.10-0.15 (medium-velocity), 0.20+ (high-velocity VAV runs). Choose lower friction for quieter systems + oversized duct; choose higher for tighter space.
How it works
We use the standard friction-loss approximation ΔP/L = 0.109136 × Q^1.9 / D^5.02 (where ΔP/L is inches water column per 100 ft, Q is CFM, D is duct diameter in inches). We invert to solve for D given a chosen friction-loss target.
Final design uses Manual D / ductulator and accounts for elbows, transitions, and dampers. This tool sizes the straight-duct ballpark.
Built by Forward
Forward runs duct sizing, CFM-for-BTU, ventilation CFM required (ASHRAE 62.1), HVAC load estimation, plus the broader NEC + IBC + IMC calcs and Procore + Autodesk + OneDrive lookups over text.