PSI vs GPM: What Actually Matters
PSI (pounds per square inch) is the force of the water; GPM (gallons per minute) is how much water flows. Multiplying them gives cleaning units. Cleaning units are a rough comparison of output, not a measure of cleaning quality.
Work out cleaning units
The calculator needs JavaScript. By hand: multiply your washer's listed PSI by its listed GPM. Compare two machines only when both figures are max or both are rated, as each maker states them.
PSI and GPM in plain terms
PSI tells you how hard the water hits. GPM tells you how much water comes out each minute. A washer lists both, and you need both to compare machines.
Cleaning units: PSI × GPM
Simpson's FAQ: "Cleaning units is calculated by multiplying PSI and GPM."
A worked example, with no particular machine in mind: 2,000 PSI × 1.2 GPM = 2,400 cleaning units. This is an estimate from listed figures. Work out your own numbers in the cleaning units calculator.
What cleaning units don't tell you
Cleaning units don't tell you which tip, distance or detergent suits a surface, and they don't measure how clean the surface ends up. They only work when both figures share a basis: a max PSI times a max GPM, or a rated PSI times a rated GPM.
Max vs rated pressure: compare like with like
Manufacturers state pressure differently: some give a maximum and a rated (working) figure, and some measure at a given nozzle. Compare like with like, as each manufacturer states it. Some examples, in the makers' words:
- Sun Joe's SPX3000 manual: "Max rating per CSA test standards. Maximum internal pressure is 2030 PSI and flow is 1.76 GPM. Under typical load, working pressure is 1450 PSI and flow is 1.24 GPM."
- Westinghouse lists its ePX3100 at "2300 max PSI and 1.76 max GPM (at lowest pressure), and 1900 rated PSI and 1.24 rated GPM."
- DeWalt gives the DWPW2100's pressure as "Maximum psi when used with 15° nozzle."
- DeWalt's gas FAQ says its "PSI and GPM values based on CETA standards are within the manufacturer's tolerance of +/- 10%."
These are the makers' statements. We name a standard only as the maker does.
More isn't always better
Use the lowest pressure that does the job. Simpson's FAQ and DeWalt's gas FAQ both say: "Household jobs… shouldn't need a machine larger than 3000 PSI" (Simpson; DeWalt).
See how to choose a pressure washer, all pressure washer picks and the nozzle guide.
FAQ
Is PSI or GPM more important on a pressure washer?
Both. PSI is the force of the water and GPM is how much flows; Simpson's FAQ multiplies them to get cleaning units. Compare both, on the same basis.
What are cleaning units?
PSI multiplied by GPM. Simpson's FAQ says: "Cleaning units is calculated by multiplying PSI and GPM." They're a rough comparison of output, not a measure of cleaning quality.
What's the difference between max PSI and rated PSI?
Manufacturers state pressure differently: some give a maximum and a rated (working) figure, and some measure at a given nozzle. Compare like with like, as each manufacturer states it.
Is higher PSI better?
No. Use the lowest pressure that does the job. Simpson's FAQ says household jobs "shouldn't need a machine larger than 3000 PSI."
Last updated: