Pressure and flow do different work
PSI describes water pressure; GPM describes water volume. Pressure helps break a bond between dirt and a surface, while flow carries loosened material away and covers area. A very high PSI number paired with low flow may clean differently from a moderate-pressure machine that moves more water.
Cleaning units are a rough comparison
Multiplying PSI by GPM produces “cleaning units,” a useful first-pass figure when ratings use comparable test methods. It does not account for nozzle efficiency, pump durability, actual operating pressure or surface suitability. Do not use the product to justify excessive pressure on delicate material.
Use operating values, not isolated maximums
Some listings emphasize maximum PSI and maximum GPM that may not occur at the same nozzle setting. Look for rated or working values and manufacturer test standards. Hose length, inlet supply, nozzle wear and altitude can affect delivered performance.
Match performance to the surface
Vehicles, wood, siding, masonry and equipment tolerate different combinations of pressure, flow, detergent and distance. Begin with a wide fan nozzle at greater distance in an inconspicuous area. Cleaning faster is not useful if it etches concrete, strips paint, forces water behind siding or damages wood fibers.
Comparison checklist
- Compare PSI and GPM together.
- Prefer rated operating figures over separate peaks.
- Check nozzle sizes supplied for the pump.
- Confirm the water source can provide required flow.
- Select for surfaces you clean most often.
Sources and further reading
- U.S. CPSC pressure-washer safety alert
- NIOSH: high-pressure injection injury warning
- Kärcher pressure-washer guidance
A pressure-washer stream can penetrate skin and cause severe injury. Never point it at a person or animal, and seek immediate medical care for an injection injury.