Nearly every machine is sold on PSI, and nearly every job is won on GPM. If you only remember one thing from this page, remember that.
What each number does
PSI is pressure. It breaks the bond between the dirt and the surface. High PSI is what chews gum off concrete and strips paint.
GPM is flow. It carries the broken-up dirt away and rinses the surface. Flow is what makes a job fast.
Think of it as a broom. PSI is how hard you push. GPM is how wide the broom is. Pushing harder with a narrow broom does not get you home sooner.
The rule most contractors settle on
Doubling your GPM roughly halves your time on a flat surface. Doubling your PSI does not, and on softer surfaces it starts doing damage. That is why crews who move from a 4 GPM machine to an 8 GPM machine notice the difference immediately, and crews who chase PSI mostly notice the extra fuel.
Rough guide by work type
- Houses, roofs, render and soft surfaces. You want flow and chemical, not pressure. This is soft washing, usually around 300 PSI with high volume. See soft wash systems.
- Driveways, patios and flatwork. 3,000 to 4,000 PSI is plenty. Spend your money on GPM instead, because 8 GPM through a surface cleaner finishes a driveway while 4 GPM is still on the first half.
- Commercial concrete, drive-throughs and grease. Flow plus heat. Pressure alone will not lift oil. See pressure washers.
- Fleet and equipment. Moderate pressure, good flow, and usually hot water.
Where people waste money
The classic mistake is buying a 4,000 PSI machine at 4 GPM for residential flatwork, then wondering why a driveway takes four hours. The same money spent on 8 GPM at 3,500 PSI would have halved the day.
The second mistake is sizing everything else to the machine and forgetting the water supply. A standard garden tap gives roughly 5 to 7 GPM. If you are running 8 GPM you need a buffer tank, or the pump will starve and cavitate. Have a look at tanks before you upgrade the machine.
Everything downstream has to match
Flow decides your nozzle sizes, your hose diameter and your surface cleaner. A 10 GPM machine pushed through a 3/8 in. hose and undersized tips will lose pressure down the line and feel slower than the machine it replaced. Match your nozzles and hose to the flow, not to the old setup.
Short version
Buy flow. Add heat if you clean grease. Only chase pressure when the job genuinely calls for it, and remember that more pressure on the wrong surface is how you end up repairing instead of cleaning.
Not sure what suits your work? Talk to a builder, we run this gear ourselves.