RectDuct · hvac thermal
RectDuct rect duct equivalent
Translate a rectangular duct size into an equivalent round diameter for velocity or fitting screens
Result
De = 1.30 · (a·b)^0.625 / (a+b)^0.25 (ASHRAE-style equivalent diameter)
How to use this estimate
Estimate equivalent round duct diameter for a rectangular duct using the ASHRAE-style relation. How it works: De = 1.30 · (a·b)^0.625 / (a+b)^0.25 (ASHRAE-style equivalent diameter). Out of scope: duct friction charts; aspect-ratio noise rules; flexible duct crush factors; mean velocity from CFM. This is an approximate screening estimate, not a substitute for manufacturer charts or a full engineering design. Does not certify code compliance; confirm manufacturer data where it applies.
Sources, methodology, and assumptions
Methodology
De = 1.30 · (a·b)^0.625 / (a+b)^0.25 (ASHRAE-style equivalent diameter)
Assumptions
Internal clear a×b; ASHRAE-style exponent form. This is an approximate screening estimate, not a substitute for manufacturer charts or a full engineering design.
Out of scope
duct friction charts; aspect-ratio noise rules; flexible duct crush factors; mean velocity from CFM
Sources
Last technically reviewed: 2026-09-11