The short version
- PSI is how hard the water hits. GPM is how much water moves each minute.
- Pressure loosens dirt; flow carries it away. You need both, but when cleaning solutions do the loosening, flow does most of the remaining work.
- More PSI isn't automatically better. The right pressure depends on the surface; too much can etch wood, damage paint or push water behind siding.
The two numbers in plain English
PSI (pounds per square inch) measures pressure, which is how hard the water strikes the surface. GPM (gallons per minute) measures flow, which is how much water the machine delivers. Popular Mechanics describes GPM as the "speed" of cleaning, because more flow means faster coverage and more thorough rinsing, and PSI as the "intensity."
A garden hose shows the relationship. Put your thumb over the end and the water shoots out harder (higher pressure) but you get a bit less of it. Take your thumb away and more water comes out with less force (Popular Mechanics).
How pressure and flow work together
Pressure washer maker Mi-T-M explains it as two jobs. Pressure is the stripping force that scrubs dirt off the surface. Flow is the rinsing power that carries the dirt away. A machine that's strong on one and weak on the other cleans unevenly: plenty of pressure with little flow loosens grime but takes a long time to flush it off.
What "cleaning units" mean
One common way to compare machines is cleaning units (CU): PSI multiplied by GPM. Mi-T-M's own example is a 3,000 PSI, 2.0 GPM machine and a 2,000 PSI, 3.0 GPM machine. Both rate 6,000 cleaning units, but the second has more rinsing power than stripping power (Mi-T-M). Popular Mechanics gives a similar example in which a lower-pressure, higher-flow machine comes out ahead (Popular Mechanics).
Why flow often matters more on house and driveway jobs
Mi-T-M says plainly that the two specs are "equally important" when you choose a machine. It also notes that GPM is usually more important to contractors: most contractors use cleaning chemicals to do the cleaning, so their job becomes mostly rinsing the dirt away (Mi-T-M).
That matches how exterior cleaning usually works:
- Siding and roofs are cleaned with a solution at low pressure (a soft wash). The solution loosens the mildew and algae, and the water rinses it off. High pressure would add risk, not cleaning power. See soft wash vs pressure wash.
- Driveways and large flat areas go faster with more flow. Popular Mechanics recommends investing in higher GPM for regular concrete cleaning and large surfaces, even if it means slightly lower PSI (Popular Mechanics).
- Reach can depend on flow too. X-Jet, the maker of a nozzle-based soft wash applicator, says how high its stream reaches depends partly on the machine's GPM (X-Jet).
Why PSI still matters
None of this means pressure is unimportant. Some dirt has to be physically stripped off, and that takes force. Pressure is also what does the damage when it's misused. Consumer Reports warns that a pressure washer's stream can damage paint and nick or etch wood and even some types of stone.
So the goal is enough pressure for the surface, not the most pressure possible. Our companion explainer, does PSI really matter?, lines up the published pressure limits for each surface. Manufacturers often set limits. James Hardie, for example, says that if you pressure wash its fiber cement siding, use a wide fan tip kept at least 6 feet from the wall and stay under 1,500 PSI (James Hardie).
How pros lower the pressure without losing flow
On a typical pressure washer the pump sets the flow, and the nozzle opening largely sets the pressure. Push the same water through a bigger opening or a wider fan and it hits more gently. Move the wand farther away and the impact drops again.
Nozzle tips are rated in degrees. Consumer Reports explains that the higher the number, the wider the spray and the less concentrated the stream (Consumer Reports). Soft wash setups go further, using low pressure plus a cleaning solution so that the chemistry, not the force, does the cleaning.
If you're shopping for your own machine
Popular Mechanics gives these typical ranges (Popular Mechanics):
| Type or task | PSI | GPM |
|---|---|---|
| Electric machines (typical) | 1,300–2,700 | 1.1–2.3 |
| Gas machines (typical) | 2,500–4,000+ | 2.0–4.0 |
| Car washing | 1,500 or less | 1.4–1.6 |
| Deck cleaning | 1,500–2,500 | 2.0+ |
| Concrete and driveways | 2,500–3,000+ | 2.5+ |
Sources don't always agree. For wood decks, Consumer Reports suggests a gentler 800 to 1,000 PSI and testing a hidden spot first (Consumer Reports). When in doubt, start low.
Safety note: A pressure washer stream can injure people. Consumer Reports recommends against using a 0-degree (red) nozzle at all, because the concentrated stream can pierce surfaces, including protective boots, and advises hearing protection, safety goggles, protective footwear and gloves (Consumer Reports). Never point the wand at a person or pet.
What this means for ACE's equipment
ACE runs a 3,000 PSI, 5.5 GPM pressure washer. That's more flow than the 2.0 to 4.0 GPM range Popular Mechanics lists for typical gas models, so more water reaches the surface every minute for rinsing. We pair it with a 20-inch surface cleaner for flat concrete and an X-Jet soft wash system, so siding and shingle roofs get a low-pressure soft wash instead of high pressure.
Quick answers
Is GPM or PSI more important?
Both matter. Pressure (PSI) is the force that loosens dirt; flow (GPM) is the water that carries it away. Pressure washer maker Mi-T-M calls them equally important, but notes that contractors usually care more about GPM because cleaning solutions do much of the work and the job becomes rinsing.
What are cleaning units?
Cleaning units are PSI multiplied by GPM. A 3,000 PSI, 2.0 GPM machine and a 2,000 PSI, 3.0 GPM machine both rate 6,000 cleaning units, but the second moves more water and rinses faster.
Does more PSI clean better?
Not always. More pressure strips harder, but it can also etch wood, damage paint and force water behind siding. The goal is enough pressure for the surface, with enough flow to rinse it quickly.
Can you lower the pressure without losing flow?
Yes. A wider spray tip, more distance from the surface or a soft wash nozzle spreads the same water over a bigger area, so it hits more gently while the pump keeps moving the same amount of water.
What size pressure washer does ACE use?
ACE runs a 3,000 PSI, 5.5 GPM pressure washer with a 20-inch surface cleaner for flat concrete and an X-Jet soft wash system for low-pressure washing of siding and roofs.
Sources
- Which is more important PSI or GPM? (Pressure Washer FAQ), Mi-T-M Corporation. https://www.mitm.com/consumer/pressure-washers/
- GPM and PSI: How to Pick the Right Pressure Washer Based on Its Cleaning Power, Popular Mechanics. https://www.popularmechanics.com/home/lawn-garden/a65012011/gpm-vs-psi-pressure-washer/
- Can You Pressure-Wash It?, Consumer Reports. https://www.consumerreports.org/home-garden/pressure-washers/surfaces-safe-to-clean-with-a-pressure-washer-a5445180933/
- Siding Cleaning and Maintenance (Technical Bulletin), James Hardie. https://assets.ctfassets.net/dzi2asncd44t/1mcW7pxuTmKq1nWLOgTRbW/40f30c33fe5a8f9b9ea5ffdbffae86ae/20-james-hardie-siding-cleaning-and-maintenance.pdf
- The X-Jet M5, X-Jet. https://xjetnozzle.com/the-x-jet-m5/
Links checked October 19, 2026. This page is general information, not a substitute for your manufacturer's instructions or a product label.