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Duct Shape Converter

The cross-sectional Area of a duct is a determining factor in duct sizing for how much airflow the duct is designed for at a specific allowable friction or noise level.

A rectangular duct of a certain width and height can be equated with an equivalent round duct of diameter where both shapes of ducts share the same cross-sectional area.

Similarly, a round duct of a specific diameter can be equated with an equivalent rectangular duct of which the combination of width and height gives the same cross-sectional area as that of the round duct.

The formula for the computation of rectangular and round duct size:


Equating the formulae for rectangular and round duct’s size:


A downloadable Excel File for Rectangular to Round / Round to Rectangular Duct Converter is available below.

Duct Converter (Excel)

Toward the end of the calculator, the Aspect Ratio of a duct is the Ratio of Width to Height or Height to Width (longer side to shorter side) of a Rectangular Duct. To calculate Total Duct Surface Area based on a duct's dimension and aspect ratio, refer to the page Rectangular Duct Aspect Ratio Calculator.

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