Pressure Washer GPM & PSI Flow Rate Calculator
Calculate Cleaning Units (CU), nozzle orifice size, and flow characteristics for your pressure washer using GPM and PSI values.
Formulas Used
1. Cleaning Units (CU):
CU = GPM × PSI
2. Hydraulic Power (HP):
HP = (PSI × GPM) / 1714
(1 HP = 550 ft·lbf/s; 1 gallon = 231 in³; conversion yields 1714 PSI·GPM per HP)
3. Nozzle Orifice Area (Torricelli / Discharge Equation):
Q = Cd × A × √(2gΔP/ρ)
In practical units: A (in²) = GPM / (29.84 × Cd × √PSI)
where 29.84 is the unit-conversion constant for water at standard conditions
4. Nozzle Orifice Diameter:
d = √(4A / π)
5. Exit Velocity:
v = Cd × √(2 × g × h)
h (ft) = PSI × 144 / 62.4 (pressure head of water)
g = 32.174 ft/s²
6. Nozzle Size Code (Industry Standard):
Size = GPM × √(1000 / PSI) × 10, rounded to nearest 0.5
Assumptions & References
- Water is assumed to be at standard conditions: density 62.4 lb/ft³ (8.34 lb/gal), temperature ~60°F (15.6°C).
- The discharge coefficient Cd equals the user-supplied nozzle efficiency (default 95%), representing real-world orifice losses.
- The constant 29.84 in the nozzle area formula is derived from unit conversion of the Torricelli equation for water in US customary units (GPM, PSI, in²).
- Cleaning Units (CU = GPM × PSI) is the industry-standard metric for comparing pressure washer effectiveness (PWNA – Pressure Washers North America).
- Hydraulic power formula: 1 HP = 1714 PSI·GPM, derived from 1 HP = 550 ft·lbf/s and 1 gallon = 231 in³.
- Nozzle size codes follow the industry convention where the two-digit number represents GPM × √(1000/PSI) × 10 (e.g., size "25" at 1000 PSI = 2.5 GPM).
- Pressure categories follow PWNA and consumer product classification standards.
- This calculator assumes a single-orifice nozzle; turbo/rotary nozzles have different effective flow characteristics.
- References: PWNA Industry Standards; Hydraulic Institute Engineering Data Book; Bernoulli/Torricelli flow equations.