Fire Flow 

When calculating fire flow, two common formulas are used. The Iowa Fire Flow formula estimates flow for fire control in the largest open area, not full extinguishment. The NFA formula applies to interior attacks in structures with ceilings ≤10 ft. Both help estimate needed fire flow during pre-planning, though results may vary. These formulas use length, width, and height of the structure, measured in feet.

Iowa Formula: (L x W x H) / 100

NFA Formula: [(L x W) / 3] x % Involvement

You can also reference NFPA 1710 (2020 edition) for their recommended standards:

• Home: 300 GPM (2x 100+ GPM handlines)

• Shopping Center: 500 GPM (2x 150+ GPM handlines)

• Apartment: 300 GPM (2x 100+ GPM handlines)

• High-Rise: 500+ GPM fire floor, 250+ GPM floor above

Fog Nozzle Reaction  

The fog nozzle reaction formula can be used to calculate the amount of nozzle reaction you will experience at a given flow rate and nozzle pressure.

Read this article to understand your nozzle reaction.

NR = .0505 X GPM X √NP

NR: Nozzle Reaction in pounds of force
NP: Nozzle Inlet Pressure in PSI

Friction Loss

The friction loss formula allows you to calculate the friction loss you experience from the pump to the nozzle. This number affects the pump discharge pressure needed to achieve your desired flow rate.

FL = C(Q/100)^2 X (L/100)

FL: Friction Loss in PSI
C: Friction Loss Coefficient (depends on hose)
Q: Flow Rate in GPM
L: Hose Length in Feet

Pump Discharge Pressure (PDP) 

Pump discharge pressure, or PDP, is calculated so that you can properly pump your hose and nozzle combination to achieve your target flow rate. Improperly setting your PDP can cause you to over or under pump a nozzle, which affects flow rate, nozzle reaction, and stream reach.

PDP = NP + FL

To factor in appliances and elevation, use: PDP = NP + FL + AFL + EL

NP: Nozzle Pressure in PSI
FL: Friction Loss (hose) in PSI
AFL = Appliance Friction Loss in PSI (Default Value 25 PSI)
EL: Elevation Gain or Loss = 0.5 X H or (EL = 5 X Number of Floor Levels Above Ground Level)
H: Height in Feet

Relay Pumping 

The relay pumping formula allows you to calculate the PDP required for the attack engine in a relay pumping operation.

Twenty is an estimated value that represents the minimum amount of pressure (PSI) at the intake of the next pumper.  Some departments may choose to use a higher pressure than 20 PSI.

PDP = FL + 20

PDP: Pump Discharge Pressure in PSI
FL: Friction Loss in PSI

Smooth Bore Flow

The smooth bore flow formula helps you calculate the GPM of a smooth bore based on a constant, the diameter of your nozzle opening, and the nozzle exit pressure.

GPM = 29.7 X D² X √NP

D: Diameter of Nozzle Opening in Inches
NP: Nozzle Exit Pressure Measured with a Handheld Pitot in PSI

Smooth Bore Nozzle Reaction 

This formula gives you the ability to calculate the nozzle reaction you will experience for a given exit pressure of a smooth bore.

NR = 1.57 x D² x NP

NR: Nozzle Reaction in Pounds of Force
D: Diameter of Nozzle Opening in Inches
NP: Nozzle Exit Pressure Measured with a Handheld Pitot in PSI