Duct design
Equivalent Duct Diameter Calculator
Estimate an equal-friction round equivalent for a rectangular duct.
Calculator
How to use
Enter the inside rectangular dimensions. This calculation approximates a round duct with comparable friction behavior, not identical area or velocity.
Formula
Dₑ = 1.30 × (W × H)^0.625 ÷ (W + H)^0.25
Variables
W and H are rectangular duct dimensions in inches; Dₑ is equivalent round diameter in inches.
Example
A 24 × 12 in duct has an equivalent round diameter of about 18.1 in.
Result interpretation
The relationship is an equal-friction approximation. Account for fittings, lining, roughness, velocity, and system conditions in final design.
FAQ
Is equivalent diameter the same as hydraulic diameter?
No. Hydraulic diameter is a geometry measure; the equivalent diameter here is used for friction comparison.