How nozzle sizing works
A nozzle orifice is sized against the machine it runs on. Two figures decide it — flow and pressure — and both are printed on the machine's data plate.
The number on the nozzle
An orifice number states flow. A #4.0 nozzle passes 4.0 GPM at 4000 PSI, and every orifice number on every make of nozzle is defined at that same reference pressure, which is what makes the numbers comparable between one supplier and another.
Flow rises with the square root of pressure
Flow through a fixed hole rises with the square root of pressure: quadruple the pressure and the flow doubles. The same nozzle therefore passes a different flow on a different machine, and it is the machine's own flow and pressure together that decide which orifice number matches it.
orifice = GPM × √(4000 ÷ PSI)
Reading the data plate
Both figures come from the machine. Every pressure washer carries a data plate — on the pump housing, the frame, or the motor end — stating rated flow and rated pressure.
Flow is printed as LPM or GPM. Pressure is printed as bar or PSI. Use the rated figures. A carton or a listing will often quote a peak figure the pump does not hold under continuous flow, and an orifice sized from that figure is an orifice the machine cannot fill.
The colour code
Nozzle colour states spray angle. Red is 0 degrees, yellow 15, green 25, white 40. Black is the soap and foam nozzle. Two nozzles of the same colour can carry different orifice numbers, and one orifice number is available in every angle.
The number stamped on the nozzle
The stamp carries both facts at once. The first two digits are the spray angle. What follows is the orifice number, with the decimal point removed and a leading zero added — which is why a whole orifice gives a four-digit code and a half gives five.
| Orifice | 0° | 15° | 25° | 40° |
|---|---|---|---|---|
| 2.5 | 00025 | 15025 | 25025 | 40025 |
| 3.0 | 0003 | 1503 | 2503 | 4003 |
| 3.5 | 00035 | 15035 | 25035 | 40035 |
| 4.0 | 0004 | 1504 | 2504 | 4004 |
Orifice by machine flow and pressure
| Flow | 100 bar | 120 bar | 140 bar | 160 bar | 180 bar | 200 bar | 220 bar |
|---|---|---|---|---|---|---|---|
| 5 LPM · 1.3 GPM | 2.2 | 2.0 | 1.9 | 1.7 | 1.6 | 1.6 | 1.5 |
| 6 LPM · 1.6 GPM | 2.6 | 2.4 | 2.2 | 2.1 | 2.0 | 1.9 | 1.8 |
| 7 LPM · 1.8 GPM | 3.1 | 2.8 | 2.6 | 2.4 | 2.3 | 2.2 | 2.1 |
| 8 LPM · 2.1 GPM | 3.5 | 3.2 | 3.0 | 2.8 | 2.6 | 2.5 | 2.4 |
| 9 LPM · 2.4 GPM | 3.9 | 3.6 | 3.3 | 3.1 | 2.9 | 2.8 | 2.7 |
| 10 LPM · 2.6 GPM | 4.4 | 4.0 | 3.7 | 3.5 | 3.3 | 3.1 | 3.0 |
| 11 LPM · 2.9 GPM | 4.8 | 4.4 | 4.1 | 3.8 | 3.6 | 3.4 | 3.3 |
| 12 LPM · 3.2 GPM | 5.3 | 4.8 | 4.4 | 4.2 | 3.9 | 3.7 | 3.5 |
| 13 LPM · 3.4 GPM | 5.7 | 5.2 | 4.8 | 4.5 | 4.3 | 4.0 | 3.8 |
| 14 LPM · 3.7 GPM | 6.1 | 5.6 | 5.2 | 4.9 | 4.6 | 4.3 | 4.1 |
| 15 LPM · 4.0 GPM | 6.6 | 6.0 | 5.6 | 5.2 | 4.9 | 4.7 | 4.4 |
| 16 LPM · 4.2 GPM | 7.0 | 6.4 | 5.9 | 5.5 | 5.2 | 5.0 | 4.7 |
| 18 LPM · 4.8 GPM | 7.9 | 7.2 | 6.7 | 6.2 | 5.9 | 5.6 | 5.3 |
| 20 LPM · 5.3 GPM | 8.8 | 8.0 | 7.4 | 6.9 | 6.5 | 6.2 | 5.9 |
Computed from orifice = GPM × √(4000 ÷ PSI). Stocked orifice sizes: 2.5 · 3.0 · 3.5 · 4.0.
Limits
An orifice smaller than the machine's flow raises working pressure at the pump above the figure on its plate. An orifice larger than the machine's flow drops pressure at the nozzle and the jet loses its edge. The plate is the reference for both.