How this calculator works
The estimate treats the opening as a short nozzle and water as an ideal incompressible liquid of density 1,000 kg/m3. The pressure difference is converted to pascals, which is the energy per cubic metre, and that energy becomes kinetic energy at the exit. Multiplying the exit velocity by the opening area gives the volume flow. Real pipes and fittings add friction, and a sharp-edged opening has a discharge coefficient below one, so the measured flow is usually lower than this ideal figure.
Formula
velocity = square root of (2 x pressure difference / density). flow = area x velocity. The pressure difference is in pascals, density is in kg/m3 and area is in m2, so flow is in m3/s. One US gallon per minute is exactly 0.003785411784 / 60 = 0.0000630901964 m3/s.
Worked example
At 50 psi gauge in the tank, 0 psi at the exit and a 1 inch inside diameter, the pressure difference is 344,737.86 Pa. The opening area is 0.000506707 m2, the ideal exit velocity is 26.2579 m/s and the flow is 0.0133051 m3/s, or 210.889 US gallons per minute.
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Frequently asked questions
Why is the flow lower in a real system?
Friction in the pipe, bends, valves and the shape of the opening all consume energy. The ideal Bernoulli result ignores them, so treat it as an upper bound rather than a measured flow.
Can I use this for a pump?
Only as a rough opening calculation. A pump adds energy and its curve defines the flow at each pressure, so a pump selection needs the pump curve and the system head losses.
Does the liquid density matter?
Yes. The calculator fixes water at 1,000 kg/m3. A denser liquid flows more slowly for the same pressure difference, and a less dense liquid flows faster.