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Rectangular to circular duct calculator

Go from a rectangular duct to its equivalent circular duct, or the other way round: if a given Ø does not fit in the ceiling void, find which rectangular duct has the same loss with one of its sides fixed.

Rectangular to circular

Equivalent Ø– mm
Next standard Ø up–
Hydraulic Ø– mm
Area– m²
Aspect ratio–

Circular to rectangular

Required side b– mm
Rounded to 50 mm– mm
Aspect ratio–

How it is calculated

De = 1.30 · (a·b)0.625 / (a + b)0.25Huebscher (ASHRAE): same airflow and same friction loss as the a × b rectangular duct
Dh = 2·a·b / (a + b)Hydraulic Ø: the one used to calculate friction with the actual velocity of the rectangular duct

Do not confuse them: the equivalent Ø is for choosing or replacing a duct (same airflow, same loss); the hydraulic Ø, for calculating the friction of the rectangular duct at its own velocity. To go from circular to rectangular, one side is fixed and the other is found until De matches.

Example

A 400 × 200 mm rectangular duct is equivalent to Ø305 mm (standard: Ø315), although its hydraulic Ø is 267 mm. The other way round: to replace a Ø315 with a 400 mm width you need a height of 213 mm, i.e. 400 × 250 rounding to 50 mm.

Bear in mind

  • With aspect ratios above 4:1, the rectangular duct loses more than the formula suggests and is harder to install and insulate.
  • A rectangular duct with the same equivalent Ø has a larger area than the circular one, so the air moves somewhat more slowly.
  • More perimeter means more sheet metal and more leakage surface: circular duct is usually cheaper and more airtight.