PSI to GPM Calculator

How to Calculate GPM from PSI

This PSI to GPM calculator estimates water flow rate in gallons per minute based on tank pressure, exit pressure, and pipe or nozzle diameter. It applies Torricelli's law, which relates the velocity of fluid exiting an orifice to the pressure difference driving the flow. This is useful for sizing drain lines, estimating tank discharge rates, and planning irrigation or fire suppression systems.

Enter the tank pressure in PSI, the exit or atmospheric pressure in PSI, and the inside diameter of the opening in inches. The calculator returns the estimated flow rate in GPM.

For related flow calculations, try our Gallons Per Minute Calculator, Liters Per Minute Calculator, or Pressure Converter.

The Flow Rate Formula

The calculator uses Torricelli's law for incompressible flow through an orifice:

$$v = \sqrt{\frac{2 \Delta P}{\rho}}$$

Where:

• $$v$$ = exit velocity (ft/s)
• $$\Delta P$$ = pressure difference converted to psf (psi × 144)
• $$\rho$$ = water density (62.4 lb/ft³)

Flow rate in GPM is then:

$$\text{GPM} = v \times A \times 448.83$$

Where $$A$$ is the cross-sectional area of the opening in square feet and 448.83 converts cubic feet per second to gallons per minute.

Example Calculation

A water tank at 72 psi discharging through a 2.5-inch diameter opening to atmospheric pressure (14.7 psi):

• Pressure difference: 72 - 14.7 = 57.3 psi
• Exit velocity: approximately 40.5 ft/s
• Flow area: 0.0342 ft² (2.5-inch diameter)
• Estimated flow: approximately 620 GPM

Important Limitations

This calculator assumes water at standard conditions, a sharp-edged orifice, and no friction losses in the pipe. Real-world flow rates are typically lower due to pipe friction, fittings, and discharge coefficients. Use this as a theoretical upper bound and apply a discharge coefficient (typically 0.6 to 0.85) for practical estimates.