Using the nozzle size chart below, Look for the PSI or Bar rating of your pressure washer on the top row (green). Then slide your finger down the relevant column until you match the Liters/Min rating of your High Pressure washer (shown on the far right column) The Nozzle Size required for your machine will be displayed on the far left column (blue) For example, if you had a 120 Bar machine at 12.4 l/min, then you would require a nozzle with size 5.0 or 050 orifice.
You sometimes need to round up or down, based on the pressure or flow (l/min) you're looking for. Note that the smaller the nozzle orifice size, the higher the pressure and the lower the flow rate.
What we typically see is that the home user, who's washing their car, will want to optimize their l/pm. So, if you're in between sizes round up. We usually only stock the 2.5/025 nozzles for the home user and the 5.0 /050 nozzles for the bigger industrial washing machines
While, the industrial contract cleaners will want to optimize their psi 9Bar pressure), and round down. The industrial contract cleaners are generally trying to remove paint from a parking lot, or rust from an object, so they are trying to generate more psi to help remove the paint or rust.