Flow verse Pressure

You will see it everywhere, from hardware stores to automotive shops, high-pressure cleaners advertising that you need higher pressures to clean better, which is in fact not quite true. A machine with 2000 Psi and 15L/min will clean faster & better than a machine with 2500 Psi at 10 L/min because of the higher flow rate resulting in a high cleaning power.


Flow is how much water comes out of the end of your nozzle or gun in a set amount of time, usually measured in litres per minute


Pressure is measured in Bar or pounds per square inch (PSI) and equates to how much power the unit has to clean surfaces. The PSI of a washer will determine how effectively debris will be lifted from the cleaning surface, the harder the bond, the higher the pressure required – subsequently selecting the correct machine for the application increases efficiency saving you time and money. PSI and effective working pressure or EWP should not be confused. Refer to my other blog on “What is EWP?”


Flow is the amount of water discharged by the pump. Water flow effects the time it takes to clean a surface, this flow is usually measured in litre’s per minute (L/Min). This is easily measured, if you time how long it takes to fill a 10 litre bucket with the machine running (preferably on low pressure). If it takes one minute your machine has an output of 10 L/Min or if it only takes forty seconds to fill then your unit has 15L/Min. A washer with a higher flow rate is more important than having higher PSI. Having more flow will result in a much more efficient clean than an equally powered unit with lower flow rate.


This is the most common question we get asked, “What is better pressure or flow?” and the answer is, “both” but it depends on what you are doing.

The most common use for a pressure cleaner is to clean dirt, mud, mould, stains etc, from concrete, cars, or buildings and most cases the more flow you have the more cleaning power you have.

A simple test to show you the differences in water flow is by using a garden hose. Turn the tap on full and put your finger over the end so you are increasing the pressure of the water coming out of the hose which results in a stronger stream, now wash some dirt or mud off, notice how it falls off pretty easy? Now turn your tap back until you get to have half the amount of water you had and once again put your finger over the end. You should notice it is much harder to move the dirt than before and requires more time in one area. You had roughly the same pressure, but your flow is the only thing that changed dramatically.

More pressure is important when you have to remove debris that has a strong bond with the surface, e.g. paint or rust on a metal surface.

You have to be careful when selecting machines that have 5000 Psi and higher, because they may fall into the industry term “Ultra High Pressure” If you want to know more about the difference between High-Pressure Cleaners and Ultra High-Pressure Cleaners, have a look at “The Myths on Ultra High Pressure”

So as you can see there is a different application for pressure and flow rates, and a majority of people will ask for a machine with higher pressure without taking into account the flow rate. So when you’re looking at your next pressure cleaner keep in mind, what am I cleaning?

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