A number of articles on fire flow formulas have been published in various fire service publications. While most of the data dealing with the Iowa and the ISO formulas have been correct, those describing the National Fire Academy Formula and its application have contained errors. This article attempts to clarify those issues from the perspective of one of the original developers of the NFA Formula and the designer of NFA courses during which the formula has been applied.


While working for the U.S. Navy during World War II, Keith Royer, working with Chief Lloyd Layman, devised the Iowa Formula to determine more effective methodologies for controlling fires on ships. After the war. Royer continually proved the accuracy of the formula during his 40-plus years of service at the University of Iowa Fire Services Extension.

The concepts on which the Iowa Formula (LxWx H)/1(X) is based are as follows.

  • Keep oxygen from the fire to stabilize and limit Btu production.
  • Determine the needed fire flow based on the volume of space in the largest fire area. The water required for the largest
  • space in a structure will be sufficient to handle any of the smaller areas.
  • When the space is involved in fire:

— Do not vent the space that is burning. The formula is based on absorbing the Btus being produced by converting water to steam, thereby providing an inert, cool atmosphere.

— Supply the total needed fire flow to the incident scene before starting fire attack.

— Using multiple small lines, choose exterior positions that will allow nozzle patterns to completely cover the involved space.

— Make small openings for the nozzles—e.g., break a small pane of glass or slightly open a door to keep oxygen input to a minimum.

— Nozzles are typically placed on a 30-degree fog pattern for reach and cooling.

—7 On a signal, open all nozzles simultaneously for approximately 30 seconds. Then close the nozzles.

— Vent the space and enter for final extinguishment.

Given the operational concepts, this formula works well.

The critical consideration involved in applying the Iowa Formula is the method a fire department uses to fight structure fires. Does the department, for example, leave the structure “buttoned up” until the total needed fire flow is on the fireground? Does it use multiple hoselines to deliver the needed flow and choose three (or more) distribution points to obtain full, simultaneous coverage of the fire area from the exterior?

If your department fights structure fires this way, then the Iowa Formula is made just for you. However, if you are to have success using the limited fire flow concept in the Iowa Formula, you also must use the delivery concepts on which the formula is based. Any significant openings in the structure, horizontal or vertical, will let steam escape and oxygen enter, thereby increasing Btu production, the fire flow, and the time required to darken down the fire.

In addition, a fire area larger than 10,000 square feet (100 ft. x 100 ft.) or the interior wall configuration affects the ability to get complete coverage simultaneously. Most fog nozzles on 1 ¾-in. lines have an effective 30-degree fog stream reach of less than 50 feet (disregarding partitioning). Many departments do not have the number of firefighters required to distribute up to 1,000 gpm to multiple openings through four to six hoselines just after arrival at the scene. Thereby, the 1,000-gpm cap on the effectiveness of the Iowa Formula is reasonable.

Finally, the Iowa method is not recommended for situations in which there is a life hazard potential in the structure.


In early courses, around 1979-1984, the NFA used the Insurance Services Office (ISO) Fire Flow Formula and a modification of the Iowa Formula. When the Preparing for Incident Command (PIC) course was rewritten, the following new NFA formula was developed:

Needed Fire Flow (NFF) = (Length of structure x Width of structure)/3 x % Involvement.

The full, or prefire planning, formula is NFT = [(L x W)/3 + Exposure Charge] x % Involvement.

The course developers, all experienced incident scene commanders, discussed the practicality of the Iowa Formula for normal operations. Normal operations include actions such as the following:

  • Performing primary search and rescue
  • before people die in heavy smoke conditions. In tltis instance—often before total extinguishment—water is available on scene and engine company personnel are entering the building with hoselines to push or keep the fire away from possible victims. Truck companies are doing horizontal and maybe vertical ventilation to support the primary search effort. The structure’s ability to contain steam has been violated. Extinguishment will now take greater fire flows.
  • hire attack is started by the first-arriving engine company and is reinforced with additional fire attack and backup lines as resources arrive on scene. Personnel are venting as they go, and the roof may be ventilated to remove the smoke and toxic gases to the outside.

A vast majority of fire departments in the United States employ aggressive interior fire attack as discussed above. We are taught not to attack a fire from a window or door position that will push the fire through the structure. We are taught to go inside and push the tire from the unburned back to the burned and out of the structure.

The NFA Formula requires more water for extinguishment than when we leave the building buttoned up using the Iowa Formula approach; however, when we take multiple 1 ⅛or 1 ¾-inch hoselines into a building for attack and backup, then we must supply the water that will allow all of the hoselines to flow simultaneously; to do otherwise is courting disaster. The NFA Formula provides this fire flow safety factor when compared with the Iowa Formula. This is very desirable when crews are operating inside the structure.

The PIC course developers designed several fire scene plot/floor plans showing building size and openings with varying percentages of involvement. These were duplicated during one of the development meetings and distributed to a number of the NFA students on campus. Each student was directed to show hoseline placement and size on the plot/floor plans indicating how their department would control the fire from a hoseline perspective.

These plot/floor plans were collected and analyzed. For each scenario, when the floor area was divided by the needed fire flows indicated by the students, the result averaged approximately three. This resulted in the linear equation NFF = (L x W)/3.

The parameters for the scenarios were as follows:

  • All the fire scenarios were designed to be aggressive interior firefighting operations. Fire involvement percentages above 50 percent are not normally interior fire attack situations.
  • All operations would be as they are in the real structural firefighting world: Start with a single interior attack line and increase the number of lines as needed and as resources arrive on scene.
  • The necessary primary search and ventilation would be achieved simultaneously with fire flow operations.

The NFA courses are very clear in stating that (1) the formula is designed for use in aggressive interior fire operations; (2) the formula becomes increasingly inaccurate above 50 percent involvement; (3) it is not designed for defensive, master-stream operations (though it is believed that the formula provides a reasonable fire flow recommendation for heavy-stream attacks above l.000 gpm “before too much vertical and horizontal structural integrity is lost to bum-through’’); and (4) there are other variables that affect needed fire flow. The NFA Formula is based on the largest fire area in a structure, as is the Iowa Formula.


Taking into account the operational concepts and parameters, the NFA Formula is more accurate than the Iowa and ISO formulas for the approach the American fire service uses to deliver water to a real-world structure fire.


To prove to yourself that the NFA Formula is best suited to the suppression method most commonly used by American fire departments, perform the following exercise:

1. Think of a 30 ft. x 60 ft., one-story, commercial building that is 50 percent involved.

2. Approximating scale, draw the building on a piece of paper show ing a front and back door and some side windows.

3. Before calculating a fire flow using any formula, determine how’ many and what size hoselines you would order pulled and charged to attack the fire. The fire just flashed over and structural integrity is good at this time.

4. Convert the number and size of lines to gpm. and add to determine the total fire flow.

5. Now, using both the Iowa and the NFA formulas, calculate the needed fire flow.

Remember, for this example, the percentage of involvement was 50 percent. Half the involved area translates to half the calculated fire flow from the NFA Formula. Which one is closer to the flow that would supply the number of hoselines chosen?

6. Do several more the same way for other fire areas in commercial establishments in your community.

This analysis makes neither formula wrong. It simply states that to choose the correct formula, you must understand the formula’s concepts and parameters. For the above example, the Iowa Formula yields 180 gpm (60 gpm through three nozzles to provide fire area coverage), and the NFA Formula yields 300 gpm (two l^-inch lines or three l^-inch lines).

The Iowa Formula would be correct tor our example if you leave the building “buttoned up,” did no search, and applied the water simultaneously from multiple exterior openings that would cover the fire area.

However, if you are firefighting the conventional way, the NFA Formula would be closer to your needs.

In addition, based on the article “Critical Flow Rate” by C. Bruce Edwards (Fire Engineering, September 1992), delivering water at a rate greater than the formula minimums extinguishes a fire in much less time. The NFA Formula, by calculating for fire department inefficiency, allows greater flow rates and reduced extinguishment time relative to the Iowa Formula.

The ISO and Iowa formulas do not account for the function of involved area; therefore, whether 15 percent or 50 percent of the structure is involved, their flow figures remain constant. For a 6,000-squarefoot building that is 50 percent involved, the ISO Formula yields 1,500 gpm required; the Iowa Formula 600 gpm; and the NFA Formula 1,000 gpm. For 25 percent involvement. the ISO and Iowa figures remain the same, but the NFA figure is adjusted accordingly to 500 gpm.

All three fire flow formulas are correct when used within their individual design parameters.

The ISO Formula was designed for setting insurance rates and rating fire departments. It should be used by those in that business. It is far too complex to be used at an incident scene, and it provides for the greatest fire flows—because it is an insurance industry formula.

The Iowa Formula was designed for calculating fire flows needed in compartmentalized fires, such as on ships. While the application is transferable to structure fires, it is not conducive to the w’ay most American fire departments attack structure fires.

The NFA Formula was developed from the experience of structural incident scene commanders. It provides for flows based on how the majority of the fire service attack structure fires and is the best all-around formula for the structural fire officer ordering an aggressive interior attack. As with all fire flow formulas, the resultant flows may have to be modified based on experience, common sense, and good judgment.


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