4. Seven percent of 535 BTU’s = 37 BTU’s of heat released for each cubic foot of normal air
16 ft. times 13 ft. times 8 ft. = 1,664 cu. ft.
1,664 cu. ft. times 37 BTU’s = 61,568 BTU’s
5. To raise the temperature of 1 lb. of water 1oF requires 1 BTU
62ºF to 212ºF = 150 BTU’s
6. To convert the entire 1 lb. of water (at 212ºF) to 1 lb. of steam requires the addition of 970
From 212oF water to 212ºF steam = 970 BTU’s
7. To convert 1 lb. of water (at 62ºF) to 1 lb. of steam requires 1,120 BTUs
150 BTU’s plus 970 BTU’s = 1,120 BTU’s
8. One gallon of water will absorb 9,330 BTU’s of heat when the water is completely converted to
8.33 lb. (weight of water) times 1,120 BTU’s = 9,330 BTU’s/gal.
9. Gallons of water required
61,568 BTU’s divided by 9,330 BTU’s/gal. = 6.6 gal.
6.6 gal. times 2 (apply in 30 sec.) = 13.2 GPM
13.2 GPM times .25 (safety factor) = 3.3 GPM
13.2 GPM plus 3.3 GPM = 16.5 GPM
NOTE: This method assumes that all the water is vaporized and the amount of water will only blacken the area involved. This method makes no allowance for exposures although that could be factored into the initial dimensions used in the calculations.
B. Rate-of-Flow (Cubic Foot) Formula Developed by Iowa State University
Based on the results of the theoretical method described in item “A”, this method was developed
as a pre-fire planning tool to establish the minimum amount of water application required to control
a fully involved fire in the largest single open area in a building. Additional lines for back-up and
exposure protection should be addressed based on the needs identified by the user. In theory, if the
minimum amount of water identified by the Rate-of-Flow formula can control the fire on the initial
attack, the need for additional water will be minimized, but should be planned for. Immediate fire
control with minimal resources also buys additional time for additional resources to arrive.
1. Formulas: Volume (in cubic feet) of area involved divided by 100 = GPM flow necessary for
2. Volume (in cubic feet) of area involved divided by 200 = gallons of water needed
13 ft. times 16 ft. times 8 ft. = 1,664 cu. ft.
1,664 cu. ft. divided by 100 = 16.6 GPM for 30 seconds
1,664 cu. ft. divided by 200 = 8.3 gallons of water required
C. Simplified formula developed by National Fire Academy
1. Determine the area of the structure (length times width)
2. Divide the area by 1/3 to determine GPM for fire area
3. Allow 25% for each exposure including another room, floor or structure
4. Determine percentage of involvement to determine water delivery needs
13 ft. times 16 ft. = 208 sq. ft.
208 sq. ft. divided by 3 = 69 GPM
1 interior exposure (25%) 69 GPM times 25% = 17 GPM
No exterior exposures
Total Required Fire flow 100 GPM*
* Rounded to nearest 100 GPM
Flow shown for single area or floor of involvement, must be increased for multiple areas or floors
Note the closeness of results in the methods after discounting the allowance for the interior exposure in the last method.