PSI and GPM Explained
Most buying decisions come down to PSI, because it’s the headline figure on every pressure washer’s spec sheet. But PSI on its own doesn’t tell you how quickly a machine will actually get through a job — that’s where GPM comes in, and for contract work, the combination of the two matters far more than either number alone.
What PSI and GPM Actually Measure
PSI (pounds per square inch) measures the force of the water coming out of the nozzle — essentially how hard it hits the surface. Higher PSI is better for breaking down stubborn, ingrained dirt, oil stains or thick grime.
GPM (gallons per minute) measures the volume of water flowing through the machine — essentially how much surface area you can rinse and flush clean per minute. Higher GPM means you cover ground faster and rinse away loosened dirt more effectively.
Cleaning power isn’t PSI alone. A common way contractors compare machines is cleaning units (PSI × GPM). Two machines with the same PSI can perform very differently on a real job if one has significantly more flow.
Matching the Machine to the Job
| Job type | What matters more | Why |
|---|---|---|
| Domestic patios, garden furniture | Moderate PSI, lower GPM is fine | Smaller areas, less ingrained dirt |
| Large driveways and commercial forecourts | Higher GPM | Covering more area per minute matters more than raw force |
| Removing oil, tyre marks, heavy grime | Higher PSI | Force is what breaks down stubborn residue |
| Rendered walls, roof tiles, delicate surfaces | Lower PSI (or softwashing instead) | High pressure risks stripping or damaging the surface |
| Fleet and commercial contract work | Both PSI and GPM, plus reliability for daily use | Speed and durability both affect profitability |
Why Contractors Often Move to Petrol or Diesel Machines
Domestic electric pressure washers are usually fine for occasional use, but they tend to sit at lower PSI and GPM than the machines built for daily commercial work. For a business running a pressure washer several days a week, a petrol or diesel unit typically offers higher sustained output, doesn’t rely on being near a power socket, and is built for longer continuous run times — all of which add up over a working week.
Don’t Forget the Nozzle
- A narrow, high-impact nozzle concentrates force for stubborn stains but covers less area per pass
- A wider fan nozzle spreads the same flow over more surface, better for general rinsing
- Rotary surface cleaners combine flow and pressure evenly across driveways and patios, usually giving a more consistent finish than a handheld lance alone
The nozzle and technique you use can matter as much as the PSI and GPM figures on the spec sheet, so it’s worth testing a machine with the attachments you’ll actually use day to day rather than judging purely on the numbers.
Frequently Asked Questions
Is higher PSI always better?
Not necessarily. Very high PSI can damage softer or more delicate surfaces such as render, wood or roof tiles, where a lower-pressure approach or softwashing is more appropriate.
What’s more important for large commercial jobs, PSI or GPM?
GPM tends to matter more for covering large areas quickly, while PSI matters more for breaking down stubborn, ingrained dirt. Most commercial jobs benefit from a good balance of both.
Should I choose electric or petrol/diesel for contract work?
For occasional use, electric is often sufficient. For daily commercial use, petrol or diesel machines typically offer higher sustained output and don’t depend on access to mains power.
Can I hire a pressure washer before buying?
Yes, hire is available, which is a useful way to test a machine’s output on your actual jobs before committing to a purchase.
Find the Right Pressure Washer
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