อัปเดต 2026-09-07

Air duct sizing

ASHRAE Fundamentals Ch.21

Sizes round or rectangular HVAC ductwork, converting rectangular geometry to an equivalent round diameter and reusing the Darcy–Weisbach hydraulic core from the liquid-pipe module.

Open the calculator

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Same Darcy–Weisbach + Swamee–Jain friction-factor machinery as the pressure-drop calculator, with ASHRAE Table 21-1 duct-material roughness and an equivalent-diameter conversion (Huebscher 1948) for rectangular ducts.

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Duct pressure drop (Darcy–Weisbach)

Δ P = f \frac{L}{D} \cdot \frac{1}{2} ρ v^{2} + Σ K \cdot \frac{1}{2} ρ v^{2}

identical hydraulic core to the liquid pressure-drop calculator; D is the (equivalent) duct diameter

Equivalent round diameter (Huebscher 1948)

D_{e q} = 1.30 \cdot \frac{( a b ) ^{0.625}}{( a + b ) ^{0.25}}

ASHRAE 2021 eq 21-1; a,b = rectangular duct sides (m) — gives the round duct with the same ΔP at the same flow

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