Why Flow Rate (GPM) Matters Far More Than Pressure (PSI)
If you ask any rookie what makes a pressure washer good, they will shout a big PSI number. If you ask an experienced exterior cleaning contractor, they will ask one question: "How many gallons per minute does it push?"
Here is why: Pressure (PSI) shears and breaks dirt bonds. Flow rate (GPM or LPM) pushes that broken grit off the surface. If you try to clean a concrete driveway with a 3,000 PSI machine pushing only 1.2 GPM (4.5 LPM), you will spend four hours etching tiny pencil lines because there is simply not enough water mass to flush away the mud you just blasted loose.
• LPM = US GPM × 3.78541
• US GPM = LPM × 0.264172 (or divide LPM by 3.785)
• Imperial GPM (UK/Australia) = LPM × 0.219969
Surface Cleaners: The 4-Inches-Per-GPM Law
One of the fastest ways contractors ruin their day is buying a giant 20-inch surface cleaner for a 2.5 GPM (9.5 LPM) pressure washer. Rotary surface cleaners require massive water thrust to spin their internal spray bars.
The ironclad rule in the pressure washing industry is 4 inches of deck diameter per 1 US GPM of pump flow (or roughly 1 inch of deck per 1 LPM).
- 2.5 GPM (9.5 LPM): Max 10-inch deck. A 16-inch deck will spin so slowly you will leave severe circular tiger stripes etched into the concrete.
- 4.0 GPM (15.1 LPM): Perfect match for a 16-inch commercial cleaner.
- 5.5 GPM+ (20.8+ LPM): Effortlessly spins 20-inch and 22-inch commercial cleaners, doubling your daily flatwork output.
The Garden Spigot Bucket Test
When you run commercial pumps pushing 4 GPM (15 LPM) or higher, residential outdoor spigots become your primary bottleneck. If your pump demands 4.0 GPM but the homeowner's outdoor spigot only delivers 3.2 GPM through an old 1/2-inch copper pipe, your pump will begin to cavitate.
Cavitation happens when the pump's cylinders suck in air bubbles because there is no positive water pressure feeding the inlet manifold. Those tiny air pockets implode under high pressure inside the pump head, pitting ceramic plungers and destroying check valves within hours.
How to run a 60-second Bucket Test: Take a 5-gallon bucket to the customer's outdoor spigot. Open the valve fully and start a timer. If it fills the 5-gallon mark in under 60 seconds, you have at least 5.0 GPM on tap and can safely hook up your 4 GPM machine directly. If it takes 75 seconds or longer, you must feed from a water tank or down-size your equipment.
