Most people shopping for a pressure washer look at PSI. It’s the number the box leads with. GPM is smaller, lower on the page, and it’s the one that actually matters on a big job.
What PSI does
PSI is force. Pounds per square inch, the pressure the water hits the surface with. High PSI on the right surface gets things clean fast. High PSI on the wrong surface, a wood deck, old brick, vinyl siding, and you’re looking at damage instead of clean.
Concrete can take a lot. Most other things can’t.
What GPM does
GPM is volume. Gallons per minute, how much water flows through the machine. That water carries the loosened dirt away. Low GPM means you’re breaking things loose but not rinsing fast enough, and some of it settles back. You end up making extra passes on the same section.
On a long driveway or a two-story exterior, GPM is the difference between a three-hour job and a six-hour one. The machine that moves more water per minute covers more ground, period.
Cleaning units: the number to compare
Multiply PSI by GPM and you get cleaning units (CU). It’s an imperfect number but a useful one for comparing machines without getting into pump specs.
| Machine type | PSI | GPM | Cleaning units |
|---|---|---|---|
| Consumer electric | 1,500–2,000 | 1.4–1.8 | ~2,700 CU |
| Consumer gas | 2,500–3,500 | 2.0–2.5 | ~6,250 CU |
| Pro cold water | 3,000–4,000 | 4.0–5.5 | ~16,500 CU |
| Pro hot water | 3,000–4,000 | 4.0–8.0 | ~24,000 CU |
Approximate midpoints. Actual output varies by nozzle, hose length, and water supply.
The gap between a consumer and a professional unit isn’t really PSI. Most consumer gas machines already hit 2,500 to 3,500 PSI, which is in the same range as pro equipment. The gap is GPM. And GPM is what moves the job.
Right PSI for each surface
More pressure is not better. Every material has a point where it starts to take damage. Concrete is forgiving. Everything else is less so.
| Surface | Safe PSI range | Notes |
|---|---|---|
| Concrete driveways | 2,500–4,000 | Handles high pressure well. High GPM speeds things up here. |
| Brick and pavers | 1,000–2,000 | Old mortar is softer than the brick. Keep it below 1,500 on anything with age. |
| Wood decks and fences | 500–1,200 | Detergent does the cleaning. Pressure rinses. That's it. |
| Vinyl and stucco siding | 500–1,000 | High pressure forces water behind panels. Soft wash only. |
| Roofs | Under 500 | Detergent kills the growth. Pressure is just the delivery vehicle. |
Hot water is the third variable
Most people don’t know about this one. Temperature changes what you can lift. Heat breaks down grease, oil, and biological growth that cold water won’t touch no matter how much pressure you run.
An oil stain on a driveway is a hot-water job. Cold at full pressure will smear it lighter. It won’t lift it. Same story with heavy mold on a north-facing wall or grease buildup on commercial concrete. Running more PSI at cold temperatures won’t solve it. Heat does.
What the box doesn’t tell you
Box-store machines are built to a price. The PSI number is real. The GPM is constrained because high-flow pumps cost money and add weight. A consumer gas unit at 3,000 PSI sounds capable, and it is, on a small concrete patio. On a full driveway or a two-story house exterior, the low GPM shows up in how long it takes and how many passes you make on the same spot.
A 1,500 PSI electric machine is the right tool for a patio table. We’re not saying don’t own one. But if you’ve ever rented or bought a pressure washer and walked away thinking "that took way longer than I expected," GPM is usually why.
How we set it up per job
We run a hot-water unit at 5 GPM. PSI is dialed per surface with the right tip. Concrete gets real pressure. Siding and roofs get a soft wash where detergent does the cleaning and pressure handles the rinse.
The right setting isn’t the highest one. It’s the one that cleans the surface without damaging it. That part matters more than the spec sheet.
Tell us the surface and what’s on it. Quote in one business day.