# Hydraulic Diameter of a Rectangular Tube

Written by Jerry Ratzlaff on . Posted in Fluid Dynamics

The hydraulic diameter, abbreviated as dh, is used to calculate the Reynolds Number, friction factor or for calculating pressure drop across two points.  For most geometries, the hydraulic diameter, or characteristic length, is not equal to the diameter of the shape.  The calculation below is an example of the hydraulic diameter when the shape is a non circular shape, as in the shape of a rectangle.  This equation can be used for a rectangle or a square if the width and height are equal to eachother.

 $$\large{ d_h = \frac { 4\;w\;h } { 2\; \left( w \;+\; h \right) } }$$ Where: $$\large{ d_h }$$ = hydraulic diameter $$\large{ h }$$ = height of tube $$\large{ w }$$ = width of tube