Both are equally important when it comes to deciding which pressure washer to buy. PSI stands for pound-force per square inch and measures how hard water sprays out from the wand. Gallons per minute refers to water flow rate; the higher the GPM, the more water flows out. Light-duty pressure washers have low PSI and GPM capabilities, making them best suited for tasks like washing your car or clearing away loose dirt and debris from your sidewalk, driveway, deck, and patio. Medium- and heavy-duty pressure washers have higher PSI and GPM ratings, enabling you to blast lasting buildup on your vinyl siding and stuck-on dirt from concrete driveways.