Guide · Identifying and buying
Max vs rated PSI and GPM
Pressure washer PSI vs GPM: why headline maximums are not one operating point, what rated output means, and why an accessory's rating is a limit, not its output.
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Short answer
A pressure washer's headline PSI and GPM are maximums, and they are often not one operating point. The rated figures are what it delivers while working, and they are the ones to use for nozzle size and accessory checks. An accessory's PSI and GPM figures are its own maximums: limits it can take, not output it adds.
Maximum and rated output
Maximum output is the headline. Rated output is the working figure, and it is usually lower. Sun Joe's SPX3000 manual (revision MR12) lists 2,030 PSI and 1.76 GPM maximum, and 1,450 PSI and 1.24 GPM rated working output.[1] Westinghouse lists the ePX3500 at 2,500 max PSI and 1.76 max GPM, and 2,000 rated PSI and 1.2 rated GPM.[2]
The two maximums are often not reached together. Greenworks rates the GPW2300 (Greenworks Commercial SKU 5107502VT) at 2,300 PSI at 1.2 GPM, with its 2.3 GPM maximum flow measured at only 100 PSI.[3] In Canada, Greenworks lists the GPW2303A at a maximum pressure of 2,300 PSI, a maximum flow of 2.3 GPM and a rated flow of 1.2 GPM.[4] Those figures belong to those models and regions only.
So the useful figure is an operating point: a pressure and the flow at that pressure, measured together.
How makers state their figures
Makers say different things about how they arrive at their figures. Read them as maker claims, not independent measurements:
| Machine | What the maker says |
|---|---|
| Sun Joe SPX3000 | Max per CSA test standards; 1,450 PSI at 1.24 GPM under typical load |
| CRAFTSMAN CMEPW2100 | 2,100 max PSI and 1.2 GPM, both measured with the 15-degree nozzle |
| Briggs & Stratton 020681V | 2,000 max PSI, 1.2 max GPM; a member of the Pressure Washer Manufacturers' Association with third-party performance testing |
| Kärcher residential line | Flow rates between 1.25 and 2.5 GPM, depending on the model |
CRAFTSMAN rates the CMEPW2100 at 2,100 max PSI and 1.2 GPM, both measured with the 15-degree nozzle.[5] Briggs & Stratton lists its 020681V at 2,000 max PSI and 1.2 max GPM and says it is a member of the Pressure Washer Manufacturers' Association with third-party performance testing.[6] That is the maker's statement about performance testing, not a safety certification. Kärcher says its residential washers have flow rates between 1.25 and 2.5 GPM, depending on the model.[7]
Units
- psi and bar. NIST lists one psi as 6,894.757 pascals[8] and one bar as exactly 100,000 pascals.[8] One bar is therefore about 14.5 psi, and 100 bar about 1,450 psi (our arithmetic).
- GPM. GPM uses the US gallon. NIST lists one US gallon as 0.003785412 cubic meters, about 3.785 liters.[8]
Accessory ratings are limits
An accessory's PSI figure is the most it can take, not the pressure it delivers. Two different figures appear on hoses:
- Working pressure. Flexzilla's spec table gives each hose a maximum working pressure, not a burst pressure, so the two numbers should be kept separate.[9]
- Burst pressure. ProPulse states a minimum burst pressure of three times working pressure and an impulse life of more than 100,000 cycles for its consumer UberFlex hose.[10] Those are maker ratings, and the burst figure is not an operating limit.
Every part in the line counts. Cat Pumps advises that a high-pressure system is only as strong as its weakest component, so every part must be selected carefully.[11] Compare each part's working rating with your machine's maximum, and use the rated pair to size the nozzle.
Is GPM or PSI more important?
We do not rank them. Both go into the nozzle size: Cat Pumps and General Pump publish nozzle number = GPM x the square root of (4,000 / PSI).[11][12] Both also have to sit within each accessory's limits. The nozzle size finder works an example from the SPX3000's rated pair.
Picks that fit3 products
Size the tips from your machine's rated pair first, then check the set's rating covers your machine's maximum.
More on choosing
DEWALT's DXPA45ST is a size 4.0 set rated to 4,500 PSI. SIMPSON's 80145 and 80146 are one orifice size each: read the size and rating SIMPSON states and pick the one your calculation matches. On Amazon, check that 'Ships from / Sold by' shows Amazon.com or the brand.
Five-tip set in size 4.0
DEWALT DXPA45ST 4,500 PSI quick-connect spray tips, 5 tips, size 4.0, stainless steel
SIMPSON five-tip set, smaller of two sizes
SIMPSON 80145 1/4 in quick-connect nozzle set, 5 tips, size 3.0, 3,600 PSI
SIMPSON five-tip set, larger of two sizes
SIMPSON 80146 1/4 in quick-connect nozzle set, 5 tips, size 4.0, 4,500 PSI
Research that exists but we did not read
Three industrial studies on high-pressure water cleaning exist: a 2009 Journal of Food Engineering paper on high-pressure washing of biofilms from stainless steel,[13] a 2019 Food and Bioproducts Processing paper on industrial-scale stand-off distance and water-jet break-up,[14] and a 2011 Journal of Materials Processing Technology study of the water-jet cleaning process.[15] We could read only their bibliographic records, not their findings, so we draw no advice from them. They are industrial research, not homeowner guidance.
We also could not confirm a working point Obsessed Garage reportedly gave for one of its wash-station machines, so we state none.
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Questions
What is the difference between max PSI and rated PSI?
Maximum is the headline figure; rated is what the machine delivers working. Sun Joe's SPX3000 manual lists 2,030 PSI and 1.76 GPM maximum, and 1,450 PSI and 1.24 GPM rated working output.[1]
Is GPM or PSI more important?
We do not rank them. Both decide your nozzle size through the formula Cat Pumps and General Pump publish, and both have to sit within every accessory's limits.[11][12]
How do I convert bar to PSI?
NIST lists one bar as exactly 100,000 pascals and one psi as 6,894.757 pascals, so one bar is about 14.5 psi (our arithmetic). Multiply bar by 14.5 for an approximate PSI.[8]
Is a hose's burst pressure its rating?
No. Working pressure and burst pressure are different fields. ProPulse states a minimum burst of three times working pressure for its consumer UberFlex hose, and the working pressure is the figure to compare with your machine.[10]