How much water do we need?”

   When teaching fire attack classes to beginning firefighters one of the most common questions is “how much water do I need?” I always smile and answer “enough to put the fire out.” I realize this is a Captain Obvious response but, it will always depend on several factors. The first is the fuel load. What and how much is burning? Different materials will oxidize at very different rates. We call this fuel controlled. What is the ventilation profile? Fire needs to breath. If there is not sufficient air, the fire can go into a ventilation-controlled condition. 

   Of course, the size of the structure will affect the fire as well. Big open areas will require a great deal of heat to create pyrolysis of other materials in the structure. Of course, small areas will react the opposite. As you continue to research the topic of needed fire flow, you will likely find two formulas. The first is the National Fire Academy Formula. This is Fire Flow (gpm) = (length x width) divided by three. In simple terms, total square footage divided by three is the gallons per minute. If there are multiple floors then use this number for each floor and add them up. There is also an Iowa flow formula. The Iowa State University (ISU) method was created in the 1950s after a series of fire studies in enclosed spaces. The ISU “Ideal Rate of Flow” formula is: Required fire flow (gallons per minute (gpm)) = V ÷ 100. V = the volume of the space that is on fire. 

   I feel it is important to recognize that the fuel in modern structures is much different now than when this research was done. So back to the original question of how much do we need? You must read the structure. Look at the smoke density and velocity to determine how hot the fire is burning. Look at what is visibly burning. Look at the ventilation profile. If the decision is made to make entry, be certain you are taking more than the number you have calculated. Overwhelm the fire! Set your gallonage on the most you can. You can always stop applying it when the fire is extinguished. My department sets our attack lines on 125 gpm and have determined the proper pump discharge pressure for the hose type and length. If we determine more than this could be needed, we will not hesitate to pull larger lines and nozzles. This we can discuss at another time.

Training Objectives

   Upon completion, the department should be able to…

   • Discuss the National Fire Academy Formula

   • Discuss the Iowa Fire Flow Formula

   • Calculate fire flow for examples of structures in your area.

   • Compare the differences in flows between the formulas.

   • Identify your departments polices on fire flows on attack lines.

   Scott Meinecke is a member of the Sheldon Volunteer Fire Department, Director of Safety for the Iowa Association of Electric Cooperatives, and field staff for the Fire Service Training Bureau. He can be contacted by email at





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