Cleaning Units: What it Means

The Pressure Washing industry communicates cleaning power as either “Cleaning Units”, measured by the equation  PSI x GPM (Gallons per Min), or as “Cleaning Units/hr”: PSI x GPM x hrs.
You will discover these stats in the manual of a quality pressure washer and it will give you a good indication of the viable cleaning power of a pressure washer.

While this information can provide a good idea of cleaning power, three imperative things are left out: heat, nozzle type, and chemicals.
Depending on the combination of these three factors, they can actually increase cleaning effectiveness several times over. As there is no standard that considers all five factors, only a professional can show you the full range of the cleaning ability of a system.

This article contains brief clarifications on the following topics:

Water Pressure – PSI
Gallons per Minute – GPM
Wands and Nozzles
Nozzle spray-angle applications
Water: the Universal Cleaner

Water Pressure – PSI

Water pressure from the pressure washer is measured in pounds per square inch or PSI. This pressure is the amount of force delivered to the surface being cleaned, and is the critical factor in removing contaminant from the surface being cleaned. PSI is determined by the hole size of the nozzle tip and the flow rate GPM (in the UK the standard is LPM – Litres Per Minute, but for CU calculations GPM is used).
The standard nozzle size for measurement is a #4 nozzle tip. This size delivers 0.4/gpm at 40psi and 4.0/gpm at 4,000psi. As nozzle sizes increase or decrease, PSI will follow suit.
A nozzle chart is necessary in order to determine PSI with different flow rates and nozzles sizes. Nozzle selection would depend on the task at hand and the amount of pressure the surface can take before it is damaged.
While PSI is constant at the tip of the nozzle, the pressure reduces as the distance from the target surface increases. Experienced pressure washing operatives understand how to manipulate these distances for maximum cleaning power and efficiency in time.

3,000 to 3,500 PSI power wash systems are the standard equipment in the contract pressure washing industry.
A big reason for this is that while there is not much of a price difference between 2,000 PSI and 3,000 PSI washers, there is a vast difference in cleaning power.

Gallons per Minute – GPM

Gallons per Minute (GPM or g/m) refers to the rate at which water flows from a pressure washer to the surface being cleaned (known as flow rate). Higher GPM improves cleaning ability.
Most commercial contractors use a flow rate of 4gpm or greater, because rates below this do not provide a sufficient flow to be able to clean efficiently. Commercial pressure washers typically deliver 4 to 6gpm.
While lesser flow rates may do a good job, it is ultimately more expensive as it takes longer, which in turn increases labour costs.


Heat increases the cleaning abilities of water, detergent, chemicals, and solvents.
In cleaning applications where grease, oil, and gum are involved heat, manifesting itself as hot water, breaks down these substances quickly, allowing the other products to work at peak efficiency.

Lances and Nozzles

Lances attach to the pressure hose with a gun, the trigger of which starts and stops the flow of water. There are variable pressure lances, dual-lances, standard lances, or straight-through lances available. The one to select depends on the job.

The working end of the lance is fitted with different sized tips or nozzles that provide different spray patterns.
Spray patterns range from zero to 65 degrees.

All power washing nozzles have a four or five-digit code stamped on them identifying the spray angle and orifice size.
The first two digits refer to the spray (dispersal) angle in degrees:  15 is 15 degrees, 25 is 25 degrees, and so on. The remaining digits refer to the orifice size, and are standardised in the industry. A three-digit number indicates half sizes.

Nozzle spray-angle applications

Zero-degree nozzle – (code stamp 00) provides a concentrated stream for blasting or gouging away stains. It is excellent for:

  • Cleaning overhead areas and kitchen vents
  • Removing dried and caked material from equipment and vehicles
  • Removing heavy stains and buildup from concrete
  • Removing rust and other types of oxidation

Caution:  Using this nozzle incorrectly will cause injury and serious damage. It can put a hole in your foot unless the proper footwear is worn.
Also, a zero-degree nozzle will leave tracks on the surface being cleaned. Prevent this phenomenon by using a zero-degree turbo rotating nozzle. A turbo nozzle when moved at the proper rate will leave a clean surface without tracks.

If the turbo nozzle is moved too fast, it leaves swirls that look like a stretched out Slinky. Turbo nozzles are available in fixed- degree as well as variable degree nozzles in fixed sizes.

15-degree nozzle – This provides a little wider spray and acts like a scraper. It is well-suited for:

  • Heavy duty scraping of water vehicles and equipment
  • Heavy mildew stain removal
  • Removing rust and other types of oxidation
  • Scraping grease and dirt from different surfaces and paint wood, masonry, and metal

25-degree nozzle – This is an excellent tool for sweeping away dirt and debris. Among its applications are:

  • General cleaning
  • Light mildew stain removal
  • Preparation surfaces for painting
  • Removing bacterial and algae build-up in pools
  • Roof, gutter, and downspout cleaning

40-degree nozzle – This nozzle washes large surface areas quickly. Its best uses are:

  • Light cleaning, washing, and rinsing
  • Washing and rinsing cars, pickups, boats, and other watercraft
  • Cleaning flat surfaces such as roofs, windows, patios, and driveways

65-degree nozzle – This nozzle is a wide spray pattern used mostly in spray bars (a long pipe with several nozzles in it to cover a wide area), to cover a wide area without much power. Its best uses are:

  • Spray Bars
  • Lightly washing a wide area
  • Chemical Application
  • Water Brooms

We carry a wide selection of all nozzles available, so we are fully prepared for every Pressure Washing task.
Knowing what equipment to use for different applications is the most important aspect of any Pressure Washing task, not only for high quality cleaning but also to reduce labour costs for our customers.

Water: the Universal Cleaner

A few words need to be said about water. Water is the “Universal Cleaner”, so called because it dissolves more substances than any other liquid, including acids and bases.
Pressure washing enhances the cleaning ability of water by adding:

  • Solvents and Chemicals
  • Heat
  • Abrasion (a combination of pressure, spray angle, rotating nozzles and brushing)
  • Dwell time (the length of time a detergent or solvent sits on the area(s) being cleaned)

The proper combination of these enhancers with water, and their correct application, will successfully clean almost anything.


The chemicals, solvents, and detergents used all depend on the surface being cleaned and the dirt being removed. Their proper application increases productivity and decreases labor costs.
Using them indiscriminately is a danger to the contractor and the environment. Careful reading and following of directions is crucial.

When cleaning chemicals are properly combined with flow rate, pressure, nozzle configuration, and heat, cleaning is fast and highly efficient.
Manipulating any of these five has an impact on cleaning – either positively or negatively – depending on what was adjusted.

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