Round-to-Rectangular Duct Equivalent
ASHRAE equal-friction equivalent diameter between round and rectangular duct, with an aspect-ratio flag.
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
Checked against published worked examples from: ASHRAE Fundamentals (duct design) / SMACNA (by name).
Details, formula, and sources
Formula or tableASHRAE 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.
Source and editionThe 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.
Free accessFirst-principles equal-friction equivalence (not equal-velocity). The fabrication drawing governs.
What governsEstimate. AHJ and licensed mechanical contractor govern. ACCA Manual J / D / S supersede rules of thumb.
API field namesmode, side_a_in, side_b_in, equivalent_diameter_in, aspect_ratio
- Equivalence basis equal friction, not equal velocityASHRAE Fundamentals