H2X Logo


Pressure Drop Calculator

Pressure drop is caused by friction between a fluid and the internal properties of a pipe

pipe roughness

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 from the surface of the pipe. And various factors cause this resistance (or turbulent flow) such as the internal pipe diameter.

Some factors that affect the pressure drop include:

pipe length

internal pipe diameter

elevation change in the pipe


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.

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.

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

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

Using H2X, you won’t need to use a pressure drop calculator as the process is automated.

The process that you follow when using H2X is:

Set the parameter for maximum velocity

Draw the 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 through the pipes, valves, and fittings can then be automatically calculated

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.


What happens if you have too much pressure drop?

You need to install a pump to increase the pressure in the system. If you don’t, the water coming out of the outlets will be weak (if it makes it there).

Can pressure fluctuate?

Yes, it fluctuates throughout the day. For example, if everyone showers simultaneously in an apartment building before going to work, the increased flow rate will decrease the pressure. Whereas if you showered at lunchtime, you would likely have more pressure available.

Should you calculate the pressure based on the peak flow rate?

Yes, this gives you the minimum pressure that you can expect during the worst-case scenario. You should also calculate the static pressure, which gives you the maximum pressure that you can expect during the best-case scenario (sometimes too good) when there is very limited flow through the system.

Can you have too much pressure?

Yes. If that is the case, you will need to install pressure reduction valves.

What else affects pressure?

Vertical height change also affects pressure. The higher water goes the less pressure there will be, and the lower it goes, the more pressure there will be. This is why water towers are usually at the top of a hill to avoid additional pumping. Valves and fittings also contribute to the pressure drop in a system.

Ready To Start?

H2X significantly reduces design time by up to 72% and dramatically increases quality by removing human error from the process and providing design warnings.

Get started with your free demo today!

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

You can check the verified results at the below link.


Flow rate

Pipe Length (the given length)

Using Our Pressure Drop Calculator

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



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 in the system and the need for pumps or pressure reduction valves at certain points in the system - most importantly at the outlets.

The friction loss can be measured in:




Metre head 

Feet head