A copper pipe's GPM describes its volumetric flow rate in terms of gallons per minute. This discharge rate relates directly to the amount of pressure that drives the fluid through the pipe. The other factors you need to consider to calculate the volumetric flow rate are the pipe's dimensions and the fluid's thickness. A wider pipe carries more gallons per minute, and a longer pipe, at a fixed pressure differential, carries less. Thicker fluid, which has a higher dynamic viscosity, allows fewer gallons of flow each minute.
Things You'll Need
 Tape measure

Measure the pipe's radius.

Raise the pipe's radius to the power of 4. If its radius measures, for instance, 0.03m, 0.03 raised to the power of 4 is 8.1 x 10^7.

Multiply the result by pi, which equals approximately 3.142. 8.1 x 10^7 multiplied by 3.142 gives 2.545 x 10^6.

Multiply the previous step's answer by the pressure drop, in Pascals, across the copper pipe. If the pressure drop, for instance, is 50,000 pascals, 50,000 multiplied by 2.545 x 10^6 yields a result of 0.12725.

Divide the previous step's answer by 0.08, which describes water's viscosity. 0.12725 divided by 0.08 yields a result of 1.591.

Divide the result by the pipe's length, measured in meters. With a length, for instance, of 30m, divide 1.59 by 30 to give 0.053. This is the flow rate through the pipe in cubic meters per second.

Multiply the result by 15,850 to convert it to gpm. 0.053 multiplied by 15,850 yields 840 gallons per minute.
References
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