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PSI vs. GPM. What matters most?

October 27, 2016 by K

When choosing a pressure washer, PSI and GPM are both equally important. The PSI refers to the amount of pressure produced and GPM refers to the amount of water flow. Each SIMPSON pressure washer is designed to have the correct combination of PSI and GPM to have the most effective cleaning experience for your needs.

Imagine PSI being a “stripping” action to blast off the grime with force on the surface you’re cleaning and GPM as the “flow” to move/rinse the dirt and grime off the surface.

PSI (Pounds per Square Inch) refers to

the amount of cleaning pressure the machine can produce.

the amount of cleaning pressure the machine can produce.

the amount of cleaning pressure the machine can produce.

GPM (Gallons per Minute) is the amount of water that is coming from the machine.

SIMPSON measures this PSI/GPM combination to create Cleaning Units (CU.)

Cleaning Units (CU) is the result of multiplying the PSI by the GPM. Cleaning units gives us a measurement of machine performance (efficiency) to compare one pressure washer to another.


Pressure washer A has 3000 PSI and 2.0 GPM = 6000 Cleaning Units

Pressure washer B has 2000 PSI and 3.0 GPM = 6000 Cleaning Units

The difference? The rinsing power is greater in pressure washer B than the stripping power. Meaning, Pressure washer B will clean (rinse dirt away) faster  than pressure washer A.  Although the CU is the same in both pressure washers, the higher the GPM, the faster the job will be finished.


Imagine you have a baseball and bowling ball flying toward a surface at 100mph. Which ball has more impact on the surface? The bowling ball, of course! The speed of the bowling ball and baseball (100mph) represents the PSI. The size of the ball represents its GPM or cleaning impact.



Generally, contractors or professional cleaners will find that they prefer a higher GPM machine over PSI since they use cleaning solutions to loosen grime instead of relying fully on PSI, their job primarily becomes one of rinsing the dirt away. The higher the GPM the faster it will be to clean large surfaces.



This is not to say that PSI isn’t an important factor to look at when deciding on the right pressure washer for your needs. Ask a SIMPSON professional what will be best for your cleaning tasks before making your final decision on choosing a pressure washer!

2 responses to “PSI vs. GPM. What matters most?”

  1. Kevin Brown says:

    I have a washer now that is 3500psi 5.5gpm. I am thinking about buying a 3000psi 8gpm. I mostly do large concrete areas. I run 150ft high pressure hose on my 24in surface cleaner. My question is will I loose any cleaning power by dropping down to 3000psi. The engine on the one I am looking at is a Honda 690. The 3500 psi is a Honda 630. Thanks for your answer.

    • SIMPSON says:

      Hi Kevin,

      You will not lose any power because you are increasing your water volume. As it is a sizeable water volume increase you will actually have more cleaning power. 24000 cleaning units versus 19450 cleaning units.


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