# How to Calculate 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.