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Calculation Machine

Equivalent Duct Diameter Calculator

Round equivalent of a rectangular or oval duct.

Reviewed by James Whitfield · How we check our maths

Your values

ⓘ Live results

Results
Equivalent round diameter
13.48 in
Rectangular area
1.111 sq ft
Aspect ratio
2.5 : 1
Round duct of equal area
14.27 in

Working

  1. 11.3 x (20 x 8)^0.625 / (28)^0.25 = 13.48 in

Quick answer

Equivalent diameter matches friction loss, not area, and uses the Huebscher relation.

The formula

De = 1.3 x (a x b)^0.625 / (a + b)^0.25

Worked example

Worked example

Using the values the calculator loads with.

Inputs

  • Duct width20 in
  • Duct height8 in

Result

  • Equivalent round diameter13.48 in
  • Rectangular area1.111 sq ft
  • Aspect ratio2.5 : 1
  • Round duct of equal area14.27 in

How to use the Equivalent Duct Diameter Calculator

  1. 1Enter the inside width and height of the rectangular duct.
  2. 2Read the equivalent round diameter for friction charts.
  3. 3Keep the aspect ratio under 4:1 where you can — flatter duct wastes energy.

Frequently asked questions

Why not just match area?

A flat duct has more wetted perimeter, so it rubs more air; equal-friction diameter is always smaller than equal-area diameter.

Does this work for flat oval?

It is a close approximation; for exact flat-oval sizing use the manufacturer's table.

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