Do not mix diameter definitions
Rectangular Duct Diameter Definitions
Compare the ASHRAE equal-friction round equivalent with hydraulic diameter and equal-area diameter. These quantities answer different questions and are generally not interchangeable.
Geometric comparison
ASHRAE Handbook, Duct Design defines De as the circular equivalent of a rectangular duct for equal airflow, resistance and length. Its SI Table 3 gives 378 mm for 400 × 300 mm; the equation returns 377.7088 mm before rounding.
Hydraulic diameter uses flow area and wetted perimeter. A NASA duct-flow derivation gives Dh=4A/Pw and, for a rectangle, Dh=2ab/(a+b). It is the characteristic length used in Reynolds/friction correlations when that model is applicable; it is not the ASHRAE equal-friction round size.
DA is only the round diameter with the same cross-sectional area. Equal area does not imply equal resistance. De alone also does not calculate pressure drop: the selected duct method still needs flow, length, roughness, fluid properties and applicable fitting losses.
Removed unsupported claims: universal aspect-ratio recommendations and an accuracy percentage without an applicability range/source. Presets and prefilled dimensions were removed so the page opens without a hidden design assumption.
