How to Calculate the Fire Pump GPM From Draft


A fire pump drafts water by sucking it from a reservoir or other body of water. This lets firefighters put out fires in rural areas, far from any hydrant or other piped supply. The main fire pump creates a vacuum; atmospheric pressures forces water from a source through a pipe and out a nozzle. A secondary pump adds extra pressure, forcing out greater quantities of water. When testing a pumps, technicians measure flow experimentally using pilot tubes or flow meters. You can calculate the theoretical flow velocity, however, from hose conditions and the pressure at the nozzle.

Things You'll Need

  • Flow-meter
  • Find the square root of the nozzle's pressure, measured by a flow-meter in pounds per square inch. If water travels through the nozzle at 40 pounds per square inch: √40 = 6.32.

  • Square the nozzle's diameter, measured in inches. If the nozzle has a 1.5-inch diameter: 1.5² = 2.25.

  • Multiply together the answers from Step 1 and 2: 6.32 x 2.25 = 14.22.

  • Multiply the product from Step 3 by 30. The conversion constant "30" relates the units you've used and factors in the density of water. With this example: 14.22 x 30 = 426.6. This answer is the hose's flow velocity, measured in gallons per minute.

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  • Photo Credit Fire Hose image by Towards Ithaca from
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