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Convert Rectangular Ducts to Round Sizes to Round Sizes Formula and Calculator

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Hydraulic and Pneumatic Engineering
HVAC Engineering

Convert Rectangular or Square Ducts to Round Sizes, Formula and Calculator

Huebscher (1948) developed the relationship between rectangular and round ducts that is used to determine size equivalency based on equal flow, resistance, and length. This relationship is defined in Equation (1),

Preview: Convert Rectangular or Square Duct to Round Sizes Calculator

Eq. 1
De = 1.30 ( a b )0.625 / ( a + b )0.250


De = circular duct diameter for equal length, fluid resistance, and airflow, in.
a = length one side of duct, in.
b = length adjacent side of duct, in.



  • ASME Code for Pressure Piping, B31.5, Refrigeration Piping and Heat Transfer Components