What is the formula for calculating GPM?

The formula for calculating GPM (gallons per minute) is expressed as GPM = 29.7 x d² x √NP. This formula is used in the context of fire service hydraulics to determine the flow of water through a nozzle based on the diameter of the nozzle (d) and the nozzle pressure (NP) in pounds per square inch (PSI).

In this formula, 'd' is the diameter of the nozzle in inches, and 'NP' represents the nozzle pressure. The factor 29.7 is a constant derived from flow characteristics of water, which ensures that the calculation yields a result in gallons per minute. The d² term signifies that flow increases with the square of the diameter, emphasizing how significant a change in nozzle size can affect water flow rates. The square root of NP indicates that flow also increases with the square root of the nozzle pressure, showing that higher pressures also contribute to increased flow rates.

This understanding is critical for driver/operators who need to ensure adequate flow rates for firefighting operations based on the equipment and circumstances they face on the scene.