How to use this calculator
- Enter orifice diameter. Use the actual bore diameter of the restriction.
- Enter pressure drop. Use the differential pressure across the orifice.
- Set Cd and density. Use a discharge coefficient and liquid density that match the installation.
- Review flow and velocity. Check volumetric flow, mass flow and jet velocity for the restriction.
How it works
Area comes from the orifice diameter:
A = pi x d^2 / 4
For an incompressible liquid, ideal velocity comes from Bernoulli:
v_ideal = sqrt(2 x dp / rho)
The discharge coefficient derates that ideal flow:
Q = Cd x A x sqrt(2 x dp / rho)
If the restriction is part of a pipe system, pair this with the pipe flow pressure drop calculator and pump NPSH calculator.
Worked example
Verified against the live calculator
A 10 mm orifice with 1 bar pressure drop,
Cd = 0.62 and water density near 998 kg/m^3
flows about 41.4 L/min, or about
10.9 gpm.
Frequently asked questions
How is orifice flow rate calculated?
For incompressible liquid flow, Q = Cd x A x sqrt(2 x pressure drop / density). The discharge coefficient accounts for losses and contraction.
What discharge coefficient should I use?
A sharp-edged liquid orifice is often around 0.60 to 0.65, but the correct value depends on geometry, Reynolds number and calibration data.
Can I use this for air or gas flow?
No. Compressible gas flow can choke and needs a gas-specific equation, pressure ratio, temperature and gas properties.
Is this a control valve sizing calculator?
No. It is a fixed-orifice liquid screen. Control valves need valve Cv/Kv data, cavitation/flashing checks and manufacturer sizing rules.
Method & assumptions
- Uses incompressible liquid flow with a user-entered discharge coefficient.
- Pressure drop is the differential across the orifice, not the total pump discharge pressure.
- Does not handle choked gas flow, steam, two-phase flow, cavitation, flashing, very low Reynolds number correction or custody-transfer metering.
- Use calibrated or manufacturer data for safety-critical restrictors, hydraulic controls and flow measurement.