Pressure Washer Guide: PSI, GPM, Nozzles, and Surface Cleaners

PSI, GPM, and Cleaning Units

PSI (pounds per square inch) measures the force of the water stream. GPM (gallons per minute) measures the volume of water. Both matter. PSI breaks the bond between dirt and surface. GPM flushes the loosened dirt away. A washer with high PSI but low GPM blasts dirt loose but does not rinse it away efficiently. The product of PSI and GPM gives you Cleaning Units (CU), which is the best single number for comparing washers.

Light-duty electric washers (1,300 to 1,900 PSI, 1.2 to 1.5 GPM) handle cars, patio furniture, grills, and light stain on concrete. They are quiet, lightweight, and connect to a standard garden hose. Adequate for homeowners who clean a few times per season.

Medium-duty gas or electric washers (2,000 to 2,800 PSI, 2.0 to 2.5 GPM) handle driveways, siding, decks, and fences. This is the sweet spot for most residential cleaning. Enough power for concrete without excessive risk of damaging softer surfaces when you use the right nozzle.

Heavy-duty gas washers (3,000+ PSI, 3.0+ GPM) are for commercial use — stripping paint, removing heavy grease from equipment, cleaning large concrete areas quickly. These will damage wood, strip paint unintentionally, and etch concrete if used carelessly. Most homeowners do not need this much power.

Nozzle Selection

Pressure washer nozzles are color-coded by spray angle. The narrower the angle, the more concentrated the force and the higher the risk of surface damage. Red (0 degrees) is a pinpoint jet — it etches concrete and strips paint. Use it only for stubborn stains on hard surfaces. Yellow (15 degrees) is for heavy-duty cleaning on concrete and brick.

Green (25 degrees) is the general-purpose nozzle — it cleans most surfaces effectively without excessive force. Use it for siding, fences, decks, and moderately dirty concrete. White (40 degrees) is for delicate surfaces — windows, painted surfaces, vehicles, and wood that you do not want to damage. Black (65 degrees) is the detergent nozzle — low pressure that lets the soap dwell on the surface.

Start with a wider nozzle than you think you need and work narrower only if the cleaning is insufficient. A 25-degree nozzle on wood siding is usually fine. A 15-degree nozzle on the same siding gouges the wood. Test on an inconspicuous area first before cleaning visible surfaces.

Turbo nozzles (rotary nozzles) spin a zero-degree jet in a cone pattern, combining the cutting power of a pinpoint stream with a wider coverage area. They clean concrete and brick 40 to 50% faster than a standard 15 or 25-degree nozzle. But they are too aggressive for wood, siding, and painted surfaces.

Surface Cleaners and Attachments

A surface cleaner attachment is a flat, circular housing with two or three spinning nozzles underneath. It rolls across flat surfaces like driveways and patios, cleaning a 12 to 15-inch swath evenly without the streaking that a handheld wand creates. For any flat surface larger than a few square feet, a surface cleaner saves time and produces better results.

Extension wands let you reach second-story siding, gutters, and high windows without a ladder. A telescoping wand extends your reach by 6 to 18 feet. The tradeoff is control — the water force at the end of a long wand makes it hard to hold steady. Start at lower pressure and work up when using extension wands.

Foam cannons attach to the washer and mix detergent into a thick foam that clings to surfaces. The foam increases dwell time — the detergent stays on the surface longer and does more cleaning before you rinse. Especially effective for vehicles, where foam loosens dirt before contact washing reduces scratch risk.

Gutter cleaning attachments are J-shaped wands that hook over the gutter edge and spray inside to flush debris. They save you from climbing a ladder to clean gutters. The spray angle is fixed, so you cannot direct it precisely, but for routine gutter flushing they work well enough.

What Not to Pressure Wash

Painted surfaces take damage from pressure washers unless you use wide nozzles (40-degree or wider) and keep distance. Pressure washing strips loose paint effectively but also strips paint that was perfectly adhered if you get too close or use too narrow a nozzle. If the goal is to keep the paint, use the gentlest setting that removes the dirt.

Old brick and mortar can be damaged by pressure washing. Mortar in older homes (pre-1930) is often lime-based and softer than modern Portland cement mortar. High-pressure water erodes soft mortar from joints. Use low pressure (under 1,500 PSI) on old masonry or hire a professional who understands the mortar type.

Windows can crack from direct high-pressure spray, especially older single-pane windows and windows with existing stress cracks. The pressure can also force water past seals and into the wall cavity. Clean windows with a low-pressure rinse or by hand. A pressure washer is not a window-cleaning tool.

Asphalt shingle roofs should never be pressure washed. The high pressure strips granules from the shingles, dramatically reducing their lifespan. Roof cleaning requires a low-pressure chemical application (soft wash) that kills algae and moss without mechanical force. If your roof has black streaks, look into soft-wash roof cleaning, not pressure washing.

Frequently Asked Questions