Saving Lives & Protecting Property
Mastering hydraulic calculations is critical for every firefighter operating on the fireground. Whether you’re calculating fire flow, friction loss, pump discharge pressure, or nozzle reaction, knowing how to apply key hydraulic formulas can make or break an operation. In this guide, we break down the essential hydraulic calculations every firefighter needs to know — from estimating fire flow to managing relay pumping and smooth bore nozzle reaction. Use these proven methods to sharpen your skills, boost fireground efficiency, and make confident decisions under pressure.
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Your calculated target flow is 0 GPM .
When calculating fire flow, 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.
NFA Formula: [(L x W) / 3] x % Involvement
You can also reference NFPA 1710 (2020 edition) for their recommended standards:
Your target flow is 0 GPM . Choose a nozzle type to open TFT NozzleAtor with the correct filter.
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
Results will appear here after calculation.
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
Results will appear here after calculation.
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 + AL + EP
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
Results will appear here after calculation.
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
Your estimated flow is 0 GPM. Open NozzleAtor with the closest smooth bore target-flow option.
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
|
Fraction
|
Decimal
|
|---|---|
|
7/8 |
0.875 |
|
15/16 |
0.938 |
|
1-1/8 |
1.125 |
|
1-1/16 |
1.063 |
|
1-3/16 |
1.188 |
|
1-1/4 |
1.25 |
Open TFT NozzleAtor filtered to Smooth Bore products.
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
|
Fraction
|
Decimal
|
|---|---|
|
7/8 |
0.875 |
|
15/16 |
0.938 |
|
1-1/8 |
1.125 |
|
1-1/16 |
1.063 |
|
1-3/16 |
1.188 |
|
1-1/4 |
1.25 |