The flow rate of water in gallons per minute, or gpm, can be calculated with the help of the Bernoulli equation and careful unit conversion. If the pressure is known in pounds per square inch, or psi, at two locations along the pipe, then the Bernoulli equation can be used to determine the velocity of the water. The Bernoulli equation states that velocity is determined by calculating difference in pressure between two points, multiplying by 2, dividing by the density of water and then taking the square root. You then get the flow rate by multiplying the velocity by the cross-sectional area of the pipe.
How to Calculate GPM from PSI for WaterBy Joshua Bush