Rectangular to Round Duct Equivalent Diameter

The equivalent diameter gives the round duct with the same friction loss and airflow.

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Formula

De = 1.30 × (a × b)^0.625 ÷ (a + b)^0.25

Example calculation

12 × 8 in → 10.7 in round equivalent.

Practical HVAC use

Use round-duct friction charts for rectangular duct.

Common mistakes

Using area-equivalent diameter instead — it underestimates friction.

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Round Duct Sizing

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Size a round duct for a target velocity from airflow, with metric output and nearest standard size.

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Estimate friction loss per 100 ft and for a duct run from airflow and round duct diameter.

Educational tool. Results are estimates from the stated formulas; always confirm with manufacturer data and the code adopted in your jurisdiction. All calculators