How to Calculate Hydraulic Diameter

Hydraulic Diameter

The hydraulic diameter is a term used in fluid mechanics and thermodynamics.  It is a measure of the efficiency of a section to pass flow by relating the cross-sectional area of flow to the wetted perimeter of the cross-section.

The hydraulic diameter of a round pipe is simply its diameter.  The equations for hydraulic diameter for other cross-sections are given in the table below.


Flowing Full
Circle D
Annulus D_{outside} - D_{inside}
Square (L = side) L
Rectangle (L1 and L2 = sides) \frac{2L_1L_2}{L_1 + L_2}
Flowing partially full
Half-filled circle (d = diameter) d
Rectangle (h = height, w = width) \frac{4hw}{w + 2h}
Wide, shallow stream (d = depth) 4d
Triangle (d = depth, t = top, s = side length) \frac{d\cdot t}{s}
Trapezoid (d = depth, t = top, b = bottom, s = side length) \frac{2d(t + b)}{b + 2s}


The hydraulic diameter should not to be confused with the hydraulic radius term in the Manning equation, which is not half of the hydraulic diameter, but rather one quarter.

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