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
Checked against published worked examples from: ACCA Manual D / ASHRAE Fundamentals.
Details, formula, and sources
Formula or tableEqual-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.
Source and editionASHRAE 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.
Free accessACCA / ASHRAE Handbook licensed; principles free in published engineering texts.
What governsEstimate. AHJ and licensed mechanical contractor govern. ACCA Manual J / D / S supersede rules of thumb.
API field namescfm, 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 galvanized steel, 0.0003 ft (fixed; the tile has no material input)public engineering reference
- Method not covered static regain; the tile sizes to a constant friction rate onlyASHRAE Fundamentals duct design chapter