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Hydraulic Diameter of Rectangular Ducts Calculator

HVAC Design and Engineering
Hydraulic and Pneumatic Design and Engineering

Hydraulic Diameter of Rectangular and Non-Circular Ducts Calculator

The characteristic length for internal flow in pipes and tubes is the inside diameter. For noncircular tubes or ducts, the hydraulic diameter Dh is used to compute the Reynolds and Nusselt numbers.

Inserting expressions for cross-sectional area and wetted perimeter of common cross sections shows that the hydraulic diameter is equal to

  • The diameter of a round pipe
  • Twice the gap between two parallel plates
  • The difference in diameters for an annulus
  • The length of the side for square tubes or ducts

For noncircular ducts Hydraulic Diameter :

Eq. 1
Dh = 4 A / P


Eq. 2
Dh = 4 (h w) / (2 h +2 w)


A = Cross-sectional area for flow (in2, mm2)
P = Total wetted perimeter (in, mm)
h = height of duct (in, mm)
w = width of duct (in, mm)


ASHRAE Fundamentals Handbook, 2021