Round-to-Rectangular Duct Equivalent

ASHRAE equal-friction equivalent diameter between round and rectangular duct, with an aspect-ratio flag.

Run the calculator

Example

You enter

Mode rect-to-round
Rect side a (in) 14
Rect side b (in) 8

You get

Equivalent diameter / other side 11.46 in
Aspect ratio 1.75:1

Details, formula, and sources

ASHRAE equal-friction equivalent diameter D_e = 1.30*(a*b)^0.625/(a+b)^0.250. Round-to-rect solves the unknown side for a target D_e; aspect ratio = long side / short side.

The ASHRAE equal-friction circular equivalent of a rectangular duct, D_e = 1.30*(a*b)^0.625/(a+b)^0.250, per ASHRAE Fundamentals (duct design) and SMACNA, by name; first-principles.

First-principles equal-friction equivalence (not equal-velocity). The fabrication drawing governs.

Estimate. AHJ and licensed mechanical contractor govern. ACCA Manual J / D / S supersede rules of thumb.

Field names used by the API: mode, side_a_in, side_b_in, equivalent_diameter_in, aspect_ratio

Equivalence basis equal friction, not equal velocityASHRAE Fundamentals