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De = 1.30 x (Len x Wid)0.625 / (Len + Wid)0.250

Where:

 

 De        = Equivalent circular diameter, in.
 Len        = Length of rectangle's side, in.
 Wid        = Width of rectangle's side, in.

 

Ref:        ASHRAE 1985 Fund. - Equation 31, Page 33.6