# Pipe Sizing Calculation – Part 4: Determining Hydraulic Diameter of the Pipe System

We will learn about hydraulic diameter calculation in this part of the water piping system design calculation.

What is Hydraulic Diameter of Pipe?

It is the equivalent diameter of a circular cross section pipe which produces same force kind of force balance as that of the non-circular cross sectional duct. It is calculated from the following equation:

Hydraulic Dia (Dh) = (4* duct cross sectional area)/ (Wetted perimeter of the duct)…………………….Eq.1

The Dh (also called as hydraulic mean diameter) is used in the Reynolds number calculation and in the Darcy–Weisbach equation.

For the water pipe sizing calculation example, we are dealing in this series,

the duct cross sectional area = π*D*D/4

the wetted perimeter of the duct = π*D

So, from the Eq.1,

Dh = D = 20 mm

Where,

D – Pipe diameter assumed in the Part-1

In the next part (Part-5), we will see how to calculate the relative roughness for the pipes.