Pressure Washer Nozzle Size Calculator
Determine the correct nozzle orifice size for your pressure washer based on your machine's flow rate (GPM) and operating pressure (PSI).
Gallons per minute output of your pressure washer pump
Pounds per square inch at the nozzle
Formulas Used
1. Water Exit Velocity (Torricelli / Bernoulli):
v = √(2 · g · P / ρ)
- g = 386.09 in/s² (gravitational acceleration)
- P = pressure (PSI = lb/in²)
- ρ = 0.03613 lb/in³ (density of water)
2. Orifice Area:
A = Q / (Cd · v)
- Q = volumetric flow rate (in³/s) = GPM × 231 / 60
- Cd = 0.90 (discharge coefficient, sharp-edged orifice)
3. Orifice Diameter:
d = √(4A / π)
4. Manufacturer Nozzle Number (industry convention):
Nozzle Number = (GPM / √(PSI / 1000)) × 100
The nozzle number is printed on the nozzle (e.g., "40" = size 4.0). Multiply by 10 to get the stamped label (e.g., 4.0 → "040").
5. Cleaning Units:
CU = GPM × PSI