H2X Tools

Pressure Drop Calculator

The pressure drop calculation looks at the friction between a fluid and the internal properties of a pipe

Pressure Drop Calculator


What is the Pressure Drop Calculation Used For and Why Is It Important?

Friction is a form of resistance that matter faces while moving over other matter. When liquid flows through a pipe, it experiences resistance stemming from the surface of the pipe. And various factors cause this resistance (or turbulent flow) such as the specified inside diameter.

Some of the most common factors include:

pipe roughness,

pipe length,

the internal pipe diameter and

the level of inclination of the pipe.

Pressure Drop Chart
Friction Loss chart

It’s important to measure this pressure loss (otherwise commonly referred to as friction loss) because it helps engineers and designers understand the overall pressure losses and the need for pressure at certain points in the system - most importantly at the outlets.

The friction loss can be measured in:




Metre head or

Feet head of the liquid or fluid

Pipe Friction Calculation Methods

There are different methods that can be used to calculate the pressure drop or friction loss as water flows through a pipe. Two of the most common methods include the Hazen-Williams calculation and the Darcy Law (or Darcy Weisbach) equation.

While the Hazen-Williams method is quite popular, there are some constraints, including that it does not take into account the fluid temperature and viscosity. It has also been reported that it loses accuracy on small pipe sizes.

The Darcy Weisbach equation appears to be the most accurate equation to calculate the friction loss or head loss in a pipe and is industry standard.

The Darcy Weisbach (or Darcy Law) Equation

The following equation can be used to calculate the pressure drop in a pipe:

Hf = f L v²/ 2gD

Components of the Equation

hf = Head loss in a pipe (m or ft)

L = Pipe length (m or ft)

V = velocity of the fluid inside the pipe (m/s or ft/s)

D = pipe inside diameter (internal diameter) (m or ft)

f = friction factor {f = [1.14 + 2 log10(D/e)]-2} and then iteratively f = {-2*log10[((e/D)/3.7)+(2.51/(Re*(f1/2))]}-2

g = gravity (m/s² or ft/s²) , which has a constant value of approximately 9.8 m/s².

Using Our Pressure Drop Calculator

The spreadsheet contains the formula that is used for the H2X calculations. The form calculates the friction loss if you have the following variables available:

Flow Rate


Pipe Length (the given length)

Friction Loss Spreadsheet

One pipe size from every pipe material available in H2X has been verified against the above spreadsheet.

You can check the verified results below.

How Can H2X Simplify Manually Calculating Pressure Drop?

Besides the fact that manually calculating friction loss is a time-consuming process, any manual calculation opens up the doors for human error.

The objective behind creating the H2X software solution was to use modern technology combined with recognised methods of plumbing engineering to automate the entire plumbing design workflow, including these calculations, so you can save hundreds of hours designing each project while reducing errors, rework, and costs.

Using H2X, you won’t need to use a manual pipe flow calculator as the process is automated.

The process is you follow when using H2X is:

Set the parameter for maximum velocity

Draw the total pipe layout

The flow rate is automatically calculated based on how many fixtures are connected to the pipe.

The pipe size is automatically calculated using the mass flow rate and maximum velocity parameter.

The pressure drop can then be automatically calculated


See what H2X can do

There's a reason our pipe sizing software is
rated 4.9/5 for customer support.

The integrated help button means that a real engineer will
assist with any design query, no matter how big or small.