Pressure washers will often be listed with two important specification ratings: GPM and PSI.
 

PSI

PSI is short for “pounds per square inch.” It is a measure of the water pressure coming out of the pressure washer. A typical garden hose dispenses water at about 50 PSI, and a coin-operated car wash provides about 700 PSI. A pressure washer, by contrast, typically dispenses water at several thousand PSI, depending upon the model.

 

GPM

GPM stands for “gallons per minute,” and describes how much water is flowing through the pressure washer.

Together, PSI and GPM provide your pressure washer’s cleaning power. PSI provides the “oomph” you need to lift dirt and grime, and GPM provides the necessary quantity of water to wash it away.

The real measure of a pressure washer’s cleaning power can be found by multiplying PSI by GPM. The higher the number, the greater the cleaning power. For example, a garden hose delivering water at 50 PSI and 5 GPM has a cleaning power of only 250. A pressure washer, however, delivering water at 2500 PSI and 2.3 GPM has a cleaning power of 5750.

Additionally, the design of the hose, nozzle, and spray gun factor into the psi rating of pressure washer. See Why does the pump have a different pressure rating than what the pressure washer is rated for?