Fire Flow Calculator

Fire Flow Calculator that finds the NFA attack flow, IFC required fire flow with its duration and tank volume, ISO needed fire flow, and the flow a fire hydrant delivers at 20 psi.

How the Fire Flow Calculator Works

The fire flow calculator has four methods, because "fire flow" means different things to a crew on scene, a plans reviewer and an insurer. NFA Tactical Fire Flow sizes the water a crew must put on a burning building. IFC Required Fire Flow gives the supply a new building must have available under the fire code.

ISO Needed Fire Flow is the figure used to grade a community's water supply for insurance. Hydrant Flow Test turns a static, residual and test-flow reading into the flow the main can deliver at 20 psi or any other target pressure.

Results show in GPM, liters per minute, cubic meters per hour or liters per second. As a hydrant fire flow calculator and an ISO fire flow calculator in one, each method also reports how many hydrants of each NFPA 291 class would cover the flow, and the step-by-step solution shows every value used.

NFA Fire Flow Formula

The National Fire Academy fire flow formula is the quick fireground estimate, and it is the default method in the fire flow calculator. Divide the area of the involved floor by three to get gallons per minute, then scale it by the share of the building on fire and by the number of floors involved.

$$\text{NFF}=\frac{L\times W}{3}\times \text{floors}\times \text{involvement}$$

Each exposure, such as a nearby building on one side, adds 25% of that flow. A 50 ft by 40 ft one-story building that is fully involved needs 2,000 / 3 = 667 GPM. With one exposure it needs 833 GPM, and at 50% involvement with no exposures it needs 333 GPM.

The tool also gives the Iowa State figure for comparison, which divides the volume of the space by 100. The same building with 10 ft walls holds 20,000 cubic feet, so Iowa State calls for 200 GPM, far below the NFA result because it assumes a fog attack in a closed room.

Involvement	NFA Flow, 50 × 40 ft	Liters per Minute
10%	67 GPM	252 L/min
25%	167 GPM	631 L/min
50%	333 GPM	1,262 L/min
75%	500 GPM	1,893 L/min
100%	667 GPM	2,524 L/min

IFC Required Fire Flow

For new buildings, IFC Appendix B sets the fire flow the water system must supply at 20 psi residual. The fire-flow area is the total floor area of all floors, except Type IA and IB buildings, which count only the three largest floors.

Table B105.1(2) then reads the flow from that area and the construction type. The first rows are below, and the tool carries the full table up to 8,000 GPM.

Fire Flow	Type IA, IB	Type IIA, IIIA	Type IV, V-A	Type IIB, IIIB	Type V-B
1,500 GPM	0 to 22,700 sq ft	0 to 12,700 sq ft	0 to 8,200 sq ft	0 to 5,900 sq ft	0 to 3,600 sq ft
1,750 GPM	22,701 to 30,200	12,701 to 17,000	8,201 to 10,900	5,901 to 7,900	3,601 to 4,800
2,000 GPM	30,201 to 38,700	17,001 to 21,800	10,901 to 12,900	7,901 to 9,800	4,801 to 6,200

Duration rises with flow. Flows up to 2,750 GPM must last 2 hours, 3,000 to 3,750 GPM must last 3 hours, and 4,000 GPM or more must last 4 hours. A 2,000 sq ft Type V-B shop needs 1,500 GPM for 2 hours, which is 180,000 gallons of stored water if the supply is a tank.

Sprinklers cut the requirement sharply under Tables B105.1(1) and B105.2. The values below match the code text reprinted in the California Fire Code, which adopts Appendix B.

Building	Sprinklers	Required Fire Flow	Duration
1-2 family, up to 3,600 sq ft	None	1,000 GPM	1 hour
1-2 family, up to 3,600 sq ft	NFPA 13D or IRC P2904	500 GPM	30 minutes
1-2 family, over 3,600 sq ft	NFPA 13D or IRC P2904	Half the table value	1 hour
Other buildings	NFPA 13	25% of table, at least 1,000 GPM	Table duration at that flow
Other buildings	NFPA 13R	25% of table, at least 1,500 GPM	Table duration at that flow

A 20,000 sq ft Type IIB warehouse needs 3,000 GPM for 3 hours without sprinklers. With an NFPA 13 system, 25% of that is 750 GPM, so the 1,000 GPM floor applies for 2 hours. Pick Commercial or Multifamily, or 1-2 Family, and the fire flow calculator applies the matching table.

NFPA 1 Fire Flow

Areas that adopt NFPA 1 instead of the IFC use Table 18.4.5.2.1, which also sets fire flow by floor area and construction type. The fire flow area is again the total floor area, with the three largest successive floors used for Type I and Type II construction, as the Osceola County NFPA worksheet lays out.

NFPA 1 cuts the required flow by 75% for a building with an approved automatic sprinkler system, but not below 1,000 GPM. The National Fire Sprinkler Association notes a further cut to 600 GPM when quick-response sprinklers protect the whole building. Separation from other buildings and lot lines earns further reductions of 25% to 40%.

ISO Needed Fire Flow

ISO uses needed fire flow to grade a community's public protection class. The formula multiplies a construction factor by an occupancy factor and adds credit for exposures and communication between buildings.

$$\text{NFF}=C\times O\times (1+X+P)C=18F\sqrt{A}$$

F is 1.5 for frame, 1.0 for joisted masonry, 0.8 for noncombustible and masonry noncombustible, and 0.6 for fire-resistive classes. O runs from 0.75 for noncombustible contents to 1.25 for rapid-burning ones. X + P is capped at 0.60, and the result is held between 500 and 12,000 GPM.

Take a one-story frame building of 5,000 sq ft with combustible contents and X + P of 0.30. Then C = 18 × 1.5 × √5,000 = 1,909, rounded to 2,000 GPM. NFF = 2,000 × 1.0 × 1.30 = 2,600, rounded to 2,500 GPM. Below 2,500 GPM the result rounds to the nearest 250, and above it to the nearest 500.

One- and two-family homes of two stories or less use a simpler table based on the distance to the nearest building, for a 1 hour duration. These values match the ISO table quoted by the University of Tennessee MTAS.

Distance to Nearest Building	ISO Needed Fire Flow
0 to 10 ft	1,500 GPM
11 to 20 ft	1,000 GPM
21 to 30 ft	750 GPM
More than 30 ft	500 GPM

A home with an NFPA 13D sprinkler system takes a different route. The ISO needed fire flow guide sets its flow as the demand at the base of the sprinkler riser, but not less than 500 GPM at 20 psi for 1 hour. Pick 1-2 Family, NFPA 13D in the tool to apply that minimum, then use the riser demand if your sprinkler design shows a higher figure.

Hydrant Flow Test at 20 psi

Used as a fire flow calculator at 20 psi, the hydrant mode starts from a field test. A hydrant flow test measures static pressure with no flow, then residual pressure while a known flow runs. NFPA 291 projects that test to the flow the main can deliver at 20 psi, the pressure fire flow is rated at.

$$Q_R=Q_F\times {\left(\frac{h_r}{h_f}\right)}^{0.54}$$

Here Q_F is the test flow, h_f is the pressure drop during the test, and h_r is the drop down to the target. With 80 psi static, 60 psi residual and 1,000 GPM flowing, h_f is 20 psi and h_r to 20 psi is 60 psi. The main can deliver 1,000 × (60 / 20)^0.54 = 1,810 GPM at 20 psi.

If you measured a pitot reading instead of a flow, the tool finds the flow from the outlet. Q = 29.83 × c × d² × √p, so a 2.5 in smooth outlet (c = 0.9) at 16 psi pitot flows 671 GPM. With the same 80 and 60 psi readings, that hydrant delivers 1,215 GPM at 20 psi.

NFPA 291 grades hydrants by the flow they deliver at 20 psi and marks each class with a bonnet color.

Class	Bonnet Color	Rated Flow at 20 psi
AA	Light blue	1,500 GPM or more
A	Green	1,000 to 1,499 GPM
B	Orange	500 to 999 GPM
C	Red	Below 500 GPM

Water Storage and Fire Pump Flow

Where no hydrant main exists, the required flow comes from a tank. Storage equals flow times duration in minutes, so 1,500 GPM for 2 hours needs 1,500 × 120 = 180,000 gallons, about 681 m³. The IFC method in the tool shows this volume on its third card, and the Tank Volume Calculator sizes the tank that holds it.

Fire pumps are rated in GPM in the US and in liters per minute elsewhere. One GPM is 3.785 L/min, so a 900 LPM pump moves about 238 GPM and an 1,800 LPM pump about 476 GPM. The Gallons Per Minute Calculator and Liters per Minute Calculator convert and time any flow.

Liters per Minute	GPM	m³ per Hour
450 L/min	119 GPM	27 m³/h
900 L/min	238 GPM	54 m³/h
1,800 L/min	476 GPM	108 m³/h
3,785 L/min	1,000 GPM	227 m³/h
5,678 L/min	1,500 GPM	341 m³/h

Pump power follows from flow and pressure. Water horsepower is GPM × psi / 1,714, so a pump delivering 1,000 GPM at 100 psi puts 58.3 hp into the water, and the motor must be larger to cover pump losses. The Pump Pressure Calculator converts head to psi, and the Hazen-Williams Equation Calculator finds friction loss in the supply main.

Fire Flow Calculator in Excel

Every method here fits in a spreadsheet. For NFA flow, =L*W/3*Floors*Involvement*(1+0.25*Exposures) returns GPM with involvement as a decimal. For a hydrant test, =Qtest*((Pstatic-20)/(Pstatic-Presidual))^0.54 returns the flow at 20 psi.

For ISO, =ROUND(18*F*SQRT(A)/250,0)*250 gives C before the class cap. The step-by-step solution in the fire flow calculator lists each value in order, so you can build a fire flow calculation spreadsheet and check it against the tool.

Fire Flow FAQs

These answers cover the fire flow questions people search most, with numbers from the fire flow calculator.

What is fire flow?

Fire flow is the rate of water, in GPM, needed to control a fire in a building, or the rate a water system must be able to supply for firefighting. Codes measure it at 20 psi residual pressure so hoses and engines still have working pressure.

On the fireground it is the attack flow crews put on the fire. In planning it is the supply a building must have, such as 1,500 GPM for 2 hours for a small wood-frame commercial building under the IFC.

How do you calculate fire flow at 20 psi?

Run a hydrant flow test and project it with Q_R = Q_F × (h_r / h_f)^0.54. With 80 psi static, 60 psi residual and 1,000 GPM flowing, the main delivers 1,810 GPM at 20 psi. Pick Hydrant Flow Test in the tool and enter the three readings.

The required fire flow at 20 psi comes from the code instead. Under IFC Appendix B, it ranges from 500 GPM for a small sprinklered home to 8,000 GPM for the largest buildings.

What is the minimum psi for a fire hydrant?

Fire flow is rated at 20 psi residual, meaning the main must hold at least 20 psi while delivering the required flow. That margin keeps positive pressure through the rest of the water system while engines draw from it.

Static pressure with no flow is usually much higher. The difference between static and residual pressure during a test shows how much more water the main can give before it falls to 20 psi.

How much water can a fire hydrant flow?

It depends on the main feeding it, not the hydrant body. NFPA 291 grades hydrants from Class C, below 500 GPM, up to Class AA, 1,500 GPM or more at 20 psi. A light blue bonnet marks a Class AA hydrant and a red bonnet marks Class C.

The tool reports how many hydrants of each class cover a required flow. A 3,000 GPM requirement needs two Class AA hydrants, three Class A or six Class B.

What are the four types of fire hydrants?

By flow, the four NFPA 291 classes are AA, A, B and C, marked light blue, green, orange and red. By design, hydrants are mainly dry-barrel types for freezing climates and wet-barrel types for mild ones.

The class matters most for planning, since it tells crews how much water to expect before they connect. Enter the hydrant's test readings in the tool to find its class.

What is the NFPA standard for fire water tanks?

NFPA 22 covers water tanks for private fire protection, NFPA 20 covers stationary fire pumps, and NFPA 291 covers hydrant flow testing and marking. NFPA 72 is a different code, the National Fire Alarm and Signaling Code.

Tank capacity is the required fire flow times its duration. A building needing 1,750 GPM for 2 hours needs 210,000 gallons, which the IFC method in the fire flow calculator works out on its third card.