Understanding PSI, GPM, and Nozzle Sizes

Using the wrong nozzle on your pressure washer can lead to two main problems: either your machine won't produce the pressure it's capable of, or you will put excessive stress on the pump, causing it to overheat or fail. Finding the correct nozzle orifice size is the only way to ensure your machine operates at its designed efficiency.

How Nozzle Sizing Works

Nozzle sizes are a standardized numerical system. By industry convention, a nozzle "size" represents the flow rate (GPM) the nozzle would allow at exactly 4000 PSI. For example, a "Size 4.0" nozzle allows 4.0 GPM at 4000 PSI. If you use that same nozzle on a 2000 PSI machine, the flow will be lower, and the pressure will be exactly 2000 PSI.

Why Size Matters

• Orifice Too Small: If the hole is too small for your pump's GPM, the pump will struggle to push the water through. This triggers the unloader valve to bypass water, creating heat and potentially burning out the pump seals.
• Orifice Too Large: If the hole is too big, the water passes through too easily. You will get the full GPM of your machine, but the PSI (cleaning power) will drop significantly below the machine's rating.

The Sizing Formula

This tool uses the square root relationship between pressure and flow:

By inputting your machine's maximum rated PSI and GPM, you find the "theoretical" perfect nozzle. In practice, you should usually round **up** to the nearest standard size (e.g., if the result is 3.2, buy a 3.5 nozzle) to protect your pump from over-pressurization.

Nozzle Degree vs. Orifice Size

It is important not to confuse the **color-coded spray angle** with the **orifice size**. The color (Red 0°, Yellow 15°, Green 25°, White 40°) only tells you the shape of the fan. Every one of those colors comes in different orifice sizes. You must check the stamp on the side of the nozzle (e.g., "25040" means 25 degrees and a 4.0 orifice).