PSI vs. GPM: Read the Whole Pressure-Washer Specification

Two numbers, two different questions

PSI means pounds per square inch: it describes pressure. GPM means gallons per minute: it describes water flow. SIMPSON defines them separately in its equipment manual, while Briggs & Stratton's buying guide considers both in comparing machines.

Think of the listing as the start of a conversation. What pressure and flow does this model deliver under its stated operating conditions? What work is it intended for? A large number on the carton does not answer whether the machine or method is appropriate for your particular siding, deck or roof.

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What a combined rating can and cannot say

Briggs & Stratton describes a cleaning-power comparison obtained by multiplying PSI by GPM. The arithmetic is useful to understand, but it is not a demonstration on your property.

For an invented illustration, 2,500 PSI multiplied by 2 GPM gives 5,000; 2,000 PSI multiplied by 2.5 GPM also gives 5,000. The same product comes from different pressure and flow figures. Neither example identifies a real machine, recommends a surface setting, or establishes how long a cleaning job will take.

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Ask for a comparison that matches your work

Our recommendation is to put the complete specifications side by side. Ask the supplier whether quoted pressure and flow are measured together, what the terms rated and maximum mean for each model, and which nozzle or attachment the figures assume.

Then read the operating and maintenance instructions before buying accessories. SIMPSON's cited December 2017 manual, for example, specifies an incoming water supply separately from its output terminology; those requirements belong to that equipment family. A purchase checklist can include water supply, power or fuel, frequency of use, service support and compatible attachments. Briggs & Stratton also identifies durability, nozzle systems and detergent equipment as considerations.

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Keep the surface decision separate

Buying a machine and deciding how to clean a material are separate decisions. Record the actual surface product and its manufacturer's care instructions alongside the equipment model. If the instructions do not support pressure washing, a favorable equipment comparison does not change that.

For a contractor explaining an estimate, this is also a clearer conversation than simply advertising more PSI: describe the approved process, the protection of the property and what is included in the job.