PSI (pounds per square inch) measures how hard water hits the surface. GPM (gallons per minute) measures how much water moves through the nozzle. Higher PSI loosens stubborn grime; higher GPM rinses a larger area faster.
Cleaning units
Multiply PSI by GPM to get cleaning units (CU). A 2,000 PSI × 1.2 GPM electric delivers 2,400 CU; a 3,200 PSI × 2.5 GPM gas machine delivers 8,000 CU — over three times the cleaning throughput.
Max vs rated pressure
Many listings show maximum pressure, which can be higher than the pressure sustained during use. Compare like with like, and treat headline numbers as an upper bound.
More isn’t always better
Excess pressure etches wood, strips paint, forces water behind siding and damages car clear-coat. Pick the lowest pressure that does the job and let detergent and flow do the rest.