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
Assumptions & References
- Water temperature assumed at 60 °F (density ρ = 0.03613 lb/in³).
- Discharge coefficient Cd = 0.90, typical for a well-formed pressure washer nozzle orifice (sharp-edged). Real values range 0.82–0.98.
- Flow is assumed incompressible and turbulent (fully developed).
- Nozzle number convention follows the Pressure Washer Manufacturers' Association (PWMA) and industry standard charts.
- Standard nozzle sizes listed: 1.5, 2.0, 2.5 … 12.0 (half-unit increments up to 7.5, then whole units).
- Cleaning Units (CU = GPM × PSI) is a common field metric for comparing cleaning power.
- References: PWMA PW-101, Bernoulli's Equation, Hydraulic Institute Standards.