What is the difference between the orifice size and the nozzle size?

The “nozzle size” number is a calculated value that considers both the flow rate (GPM) and the maximum pressure (PSI) of the pressure washer (see formula below). Nozzle size is an industry-standard nominal figure, like a score or index, used to identify compatibility. It is not a dimensional measurement. The dimensional measurement is “orifice size,” which is measured with a gage and expressed in inches or millimeters.