The pressure drop across a pipe determines how much water flows through it each minute. To accurately calculate the volumetric flow from the pressure drop, you must consider the pressure using the standard unit of Pascals rather than pounds per square inch (psi). The other relevant factors in the equation involve the pipe's dimensions, with wider, shorter pipers at a constant pressure differential producing a larger flow.

Multiply the pressure by 6,894.75 to convert it to Pascals. For example, if the pressure is 12 psi: 12 x 6,894.75 = 82,737 Pascals.

Multiply the pressure by the pipe's cross-sectional area. With a cross-sectional area, for instance, of 0.00786 square meters: 82,737 x 0.00786 = 650.31 Pascals.

Multiply the result by the square of the pipe's radius. If the radius measures, for instance, 0.05 meters in length: 650.31 x 0.05^2 = 1.626.

Divide the result by the pipe's length. With a length, for instance, of 25 meters: 1.626 / 25 = 0.065.

Divide the answer by 0.08, which describes water's viscosity: 0.065 / 0.08 = 0.8125. This is the flow through the pipe in cubic meters per second.

Multiply the result by 60: 0.8125 x 60 = 48.75. This is the flow in cubic meters per minute.

Multiply the result by 264.17: 48.75 x 264.17 = 12,878. This is the pipe's approximate volumetric flow in gallons per minute (gpm).