Converter
Round ↔ rectangular duct converter
Swap a round duct for a rectangular one — or the reverse — without changing how much air it carries or how much pressure it loses. The conversion uses the ASHRAE circular-equivalent (Huebscher) relationship, the same equal-friction basis used across the sizing console.
Enter a round diameter to see the matching rectangular sizes that hold the same friction and capacity.
How the conversion works
For a rectangle with sides a and b (in inches), the
circular-equivalent diameter is
Dₑ = 1.30 · (a·b)^0.625 / (a+b)^0.25.
A round and a rectangular duct that share the same equivalent diameter lose the
same pressure per unit length and carry the same airflow.
The catch is velocity. Because the two shapes have different cross-sectional areas, the rectangular duct runs faster than the round one it replaces — equal friction does not mean equal velocity. If a specific velocity matters, size by velocity in the console instead. And keep the aspect ratio at or below about 4 to 1: past that, sheet metal, sealing, and turbulent losses make a tall, thin duct a poor trade.
Need the underlying figure for any shape, or a flat-oval match? Use the equivalent diameter and flat-oval tools, or run the whole thing at once in the sizing console.