Hydraulic Diameter

The Hydraulic Diameter calculator computes the hydraulic diameter based on the area of the cross-section of the duct (A) and the wetted perimeter (p).

INSTRUCTIONS: Choose units and enter the following:

• (A) This is the cross-section area
• (p) This is the wetted perimeter

Hydraulic Diameter (HD):  The calculator returns the diameter in meters.  However, this can be automatically converted to compatible units via the pull-down menu.

The Math / Science

Hydraulic Diameter is used to calculate the pressure loss in a ducts or pipe.  The formula for the hydraulic diameter from the area and wetted perimeter is:

dh = 4 A / p

where:

• dh = hydraulic diameter
• A = area section of the duct 
• p = wetted perimeter of the duct 

A hydraulic diameter is useful for dimensionless quantities (e.g.  Reynolds number or Gauckler-Manning Equation) which employ a single variable for flow analysis.

Hydraulic diameter is mainly used for calculations involving turbulent flow. Secondary flows can be observed in non-circular ducts as a result of turbulent shear stress in the turbulent flow. Hydraulic diameter is also used in calculation of heat transfer in internal-flow problems.