Making Sense of Pressure Washer Terminology

Maintaining Your Pressure Washer’s Pump

Shopping for a pressure washer can be confusing. What are PSI, GPM and CU, and how do they affect cleaning? Which type of pump should you get? What do the nozzles do? Here’s what you need to know about these terms so you can pick the right equipment for your needs.

Pressure, Rate and Cleaning Power

The cleaning power of a pressure washer comes from the pressure and rate of water it sprays. This is expressed as Cleaning Units (CU), which are calculated by multiplying PSI by GPM.

Pressure is measured in pounds per square inch (PSI.) Pressure will vary depending on the nozzle used, but this rating is always the output when used with a nozzle that has a #4 (0.052”/1.32mm) orifice. This is the standard nozzle size used in most small to medium-sized pressure washers.

Rate is stated as gallons per minute (GPM.) This is the maximum volume the pump can move over time, but it’s limited by the water supply. The typical outdoor faucet pumps out water at a pressure of 40-60 PSI. Attached to a 25 foot, ½ inch diameter hose, this is enough to deliver up to 24 gallons per minute. Switch to a 100-foot hose, and the added resistance to flow reduces the supply to around 6 GPM. To get the most flexibility with this water supply, portable pressure washers are built to run high PSI to make up for low flow rates.

How does this apply when comparing pressure washers? Let’s say you’re looking at Billy Goat’s PW37A0H and the PW40S0H power washers. At first glance, there doesn’t seem to be much difference: the PW37 makes 3,700 PSI, while the PW40 makes 4,000 PSI. That’s less than a 10% difference between these two models. However, the PW37 pumps 3.5 GPM while the PW40 pumps 4 GPM. That works out to 12,950 CU and 16,000 CU, so the PW40 is actually around 25% more effective than the PW 37.

Pump Types

Two types of pumps dominate the residential and small commercial market: axial cam and triplex.

An axial cam pump uses an impeller to pull in water from the inlet and push it out of the housing. This type of pump is driven directly by the crankshaft. These pumps are cheap, but their ability to generate pressure is limited, and they have a short service life.

A triplex pump uses pistons to pull in water from the supply line and push it out into the high-pressure hose. By using three pistons that push and pull at different times, pulsations are kept to a minimum. These pumps can connect to the engine using a gear drive for greater pumping speed, and they last several times longer than axial pumps. All Billy Goat pressure washers use triplex pumps made by Annovi Reverberi or CAT.


Each nozzle has a code stamped on it. The first two digits indicate the angle of spray, while the next two or three digits indicate the orifice size. These nozzles also come in standard colors for each angle of spray, making them easy to identify at a glance. Here’s what you’ll get with a Billy Goat pressure washer:

00 (Red) – This 0-degree nozzle is used for maximum power, busting through caked-on material and heavy build-up. However, it’s so powerful that it can damage soft surfaces.

15 (Yellow) – This is often referred to as a “chiseling” tip. It’s useful for peeling off stubborn surface layers including paint and mildew, but it’s too strong for general cleaning.

25 (Green) – A wider spray lets this tip move loose contaminants and debris quickly. It’s great for removing mud, grass clippings and leaves.

40 (White) – A wide, gentle spray makes this nozzle a good choice for sensitive surfaces including cars, windows, and wooden decks.

65 (Black) – On Billy Goat pressure washers, this is a “soaping” nozzle: the low-pressure spray creates a siphon effect to pull soap through the detergent hose. It doesn’t provide enough pressure to physically remove dirt, but it does deposit soap over a wide area for fast coverage.

Getting Parts for Your Pressure Washer is a certified dealer for Billy Goat and their manufacturing partners including AR and CAT pumps so you can get anything you need for your pressure washer all at once. Our site has built-in factory parts diagrams and descriptions so you can find the right part to fix your equipment, and we can ship those parts to any address in the U.S. and Canada.

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