Rectangular to Round Duct Converter

Converts rectangular and flat oval duct to circular equivalent diameter using the Huebscher equation, and works in reverse: give it a round duct and a ceiling void depth and it lists rectangular sizes carrying the same air at the same friction rate.

Conversion

Equal-friction, equal-airflow basis.

Convert

Width a (in)

Height b (in)

Airflow — optional, adds velocity (CFM)

Equivalent Diameter Results

Circular equivalent diameter: 18.28 in
Aspect ratio: 2.00:1

Cross-sectional area: 2.000 ft²
Perimeter: 72.0 in
Hydraulic diameter 4A/P: 16.0 in
Velocity in the real section: —
Velocity in the equivalent round duct: —

Equivalent diameter is an equal-friction, equal-airflow basis — the equivalent round duct has less area than the section it represents. Use the real section velocity for noise and terminal selection.

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Important Disclaimer

This calculator is for preliminary design guidance only. Final duct sizing, construction, and installation must be performed by qualified professionals and comply with applicable codes and standards.