Duct Sizing
Round and rectangular duct size from CFM and friction rate.
Example
You enter
- CFM 400
- Friction rate (in w.c. / 100 ft) 0.08
You get
- Round diameter 10.14 in
- Equivalent square 9.28 in (square)
- Friction rate band green
Details, formula, and sources
Equal-friction sizing. Solves Darcy-Weisbach with the Colebrook-White friction factor for the round diameter whose loss equals the target friction rate, then converts to a rectangle by the Huebscher equivalent-diameter relation D_e = 1.30 (a b)^0.625 / (a + b)^0.25. Friction rate 0.08–0.10 in WC per 100 ft typical for residential systems. Static regain is NOT computed.
ASHRAE Handbook (Fundamentals), duct design chapter, by name (Darcy-Weisbach / Colebrook-White and the Huebscher relation). ACCA Manual D, 3rd edition by name for the residential friction-rate practice.
ACCA / ASHRAE Handbook licensed; principles free in published engineering texts.
Estimate. AHJ and licensed mechanical contractor govern. ACCA Manual J / D / S supersede rules of thumb.
Field names used by the API: cfm, friction_in_wc_per_100ft, round_diameter_in, equivalent_square_in, friction_color
- Default friction rate 0.08 in WC per 100 ft unless user suppliesACCA Manual D 3rd ed. typical
- Duct surface roughness data/hvac/duct-friction.json by materialpublic engineering reference
- Method not covered static regain; the tile sizes to a constant friction rate onlyASHRAE Fundamentals duct design chapter