Nozzle Flow Calculator
Work out how much flow a nozzle discharges under pressure. Enter the nozzle bore, the driving gauge pressure and (optionally) the discharge coefficient and fluid density, and the tool returns the flow rate in litres per second and cubic metres per second along with the jet exit velocity.
Enter Values
Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter the nozzle exit diameter in millimetres and the driving gauge pressure in kPa.
- Optionally set the nozzle coefficient Cd (defaults to 0.97) and the fluid density (defaults to 1000 kg/m³ for water).
- Read the flow rate, exit velocity and nozzle area, and open the steps to see each substitution.
How it works
The jet velocity from a pressure head follows Bernoulli's equation: an ideal velocity of √(2P/ρ), scaled by the discharge coefficient Cd to give v = Cd·√(2P/ρ). Multiplying by the exit area A = (π/4)·d² gives the discharge Q = Cd·A·√(2P/ρ). Pressure is converted from kPa to Pa and diameter from mm to m before substitution.
Worked example
Worked example. A 20 mm nozzle with Cd = 0.97 running water (ρ = 1000 kg/m³) at 300 kPa gauge: A = 0.000314 m², v = 0.97·√(600) = 23.76 m/s, and Q = 0.97·0.000314·24.495 = 0.00746 m³/s ≈ 7.46 L/s.
Common mistakes
- Using an orifice coefficient (~0.61) for a smooth nozzle — nozzles are much closer to Cd ≈ 0.95–0.98.
- Entering pressure in bar or psi instead of kPa, or forgetting it is gauge pressure.
- Reading the diameter as the upstream pipe bore rather than the nozzle exit diameter.
Frequently asked questions
What discharge coefficient should I use?
A well-formed converging nozzle is typically Cd ≈ 0.95–0.98; use the manufacturer's rated value when you have it. Sharp-edged orifices are far lower (~0.61) because of the vena contracta.
Does this include pipe friction upstream?
No. It uses the gauge pressure at the nozzle as the driving head and ignores approach velocity and upstream line losses, so feed it the pressure available right at the nozzle.
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