Fire Pump Sizing Calculator
How the formula works
The calculator adds the sprinkler or hydrant flow and the hose allowance, finds the pump head, then calculates the power and a standard motor size.
Formulas
Design flow = Sprinkler / hydrant flow + Hose allowance + Additional flow
Pump head (m) = (Residual pressure + Friction loss) in bar × 10.197 + Static height − Water level above pump
Hydraulic power (kW) = Flow (m³/h) × Head (m) × 0.0027241
Shaft power = Hydraulic power / Pump efficiency
Motor power = Shaft power × (1 + Motor margin)
Where:
- Static height = height from the pump centre to the highest outlet
- Water level above pump = positive if the tank water is above the pump, negative if below
- 1 gpm = 3.785 L/min; 1 bar = 10.197 m of water
Typical values
Sprinkler flow and hose allowance (gpm): light hazard 150 + 100, ordinary hazard group 1 225 + 250, group 2 300 + 250, extra hazard group 1 750 + 500. Residual pressure: sprinkler 1.4 bar, hydrant 3.5 bar, standpipe hose 6.9 bar. Standard fire pump ratings follow NFPA 20 (for example 250, 500, 750, 1000, 1500 gpm). These are typical values. The local code and the authority having jurisdiction are final. Use the Custom option for your own values.
NFPA 20 pump curve limits
The pump must give at least 65 % of rated head at 150 % of rated flow, and the shut-off head must not exceed 140 % of rated head. The calculator shows both limits.
How to use the calculator: select the hazard, enter static height and friction loss, choose residual pressure, efficiency and motor margin, then click Calculate.
Important notes: jockey pump, diesel engine drive and standby pumps are not sized here.
Worked example
Example 1: Light hazard building
150 gpm sprinkler + 100 gpm hose, static height 30 m, friction loss 0.8 bar, sprinkler residual 1.4 bar, efficiency 65 %, motor margin 15 %.
Flow = 250 gpm = 946 L/min (standard rating 250 gpm). Head = (1.4 + 0.8) × 10.197 + 30 = 52.4 m (5.14 bar). Hydraulic power = 8.11 kW. Shaft power = 12.48 kW. Motor = 14.35 kW. Standard motor: 15 kW.
Example 2: Ordinary hazard group 2
300 gpm sprinkler + 250 gpm hose, static height 60 m, friction 1.5 bar, hydrant residual 3.5 bar, efficiency 70 %, margin 15 %.
Flow = 550 gpm = 2,082 L/min, so the standard rating is 750 gpm (2,839 L/min). Head = 111.0 m (10.88 bar). Hydraulic power = 51.50 kW. Shaft power = 73.57 kW. Motor = 84.61 kW. Standard motor: 90 kW.
Example 3: Extra hazard, tall building
750 gpm sprinkler + 500 gpm hose + 500 L/min additional, static height 90 m, friction 2 bar, standpipe residual 6.9 bar, water level 3 m above pump, efficiency 70 %, margin 10 %.
Flow = 5,232 L/min (1,382 gpm), standard rating 1500 gpm. Head = (6.9 + 2) × 10.197 + 90 − 3 = 177.8 m (17.43 bar). Hydraulic power = 164.97 kW. Shaft power = 235.67 kW. Motor = 259.24 kW. Standard motor: 315 kW.
Example 4: Custom flow
2,000 L/min hydrant flow + 950 L/min hose, static height 20 m, friction 0.5 bar, hydrant residual 3.5 bar, water level 2 m below pump, efficiency 65 %, margin 20 %.
Flow = 2,950 L/min (779 gpm), standard rating 1000 gpm. Head = 62.8 m (6.16 bar). Hydraulic power = 38.85 kW. Shaft power = 59.77 kW. Motor = 71.72 kW. Standard motor: 75 kW.
Common mistakes
- Forgetting the hose stream allowance and sizing the pump for sprinkler flow only.
- Ignoring the residual pressure needed at the most remote sprinkler or hydrant.
- Using pipe friction loss at the wrong flow instead of the design flow.
- Mixing up metres of head, bar and psi.
- Measuring static height from the tank water level instead of from the pump centre.
- Choosing a pump size smaller than the design flow instead of the next standard rating.
- Forgetting the 150 % flow and 140 % shut-off curve limits.
- Sizing the motor at the duty point only, with no margin for the pump curve.
- Adding the jockey pump flow into the main pump rating.
Frequently asked questions
How do I calculate fire pump flow?
Add the sprinkler demand, the hose stream allowance and any other fire water demand. Then choose the next standard pump rating.
What are standard fire pump sizes?
NFPA 20 lists ratings such as 250, 500, 750, 1000, 1500, 2000 and 2500 gpm.
How is fire pump head calculated?
Add the residual pressure, the pipe friction loss and the static height. Subtract any positive suction head.
What is the residual pressure?
It is the pressure still needed at the most remote sprinkler or hydrant outlet while flowing.
How do I convert gpm to L/min?
Multiply gpm by 3.785. For example, 500 gpm is 1,893 L/min.
How do I convert bar to metres of head?
Multiply bar by 10.197. One bar is about 10.2 m of water.
What is the 150 % rule?
At 150 % of rated flow the pump must still give at least 65 % of rated head.
What is the shut-off head limit?
The head at zero flow must not exceed 140 % of rated head.
How is the fire pump motor size found?
Hydraulic power is divided by efficiency, then a margin is added and the next standard motor size is chosen.
Does this include the jockey pump?
No. The jockey pump keeps the line pressurised and is sized separately for leakage.
How big should the fire water tank be?
Use the Fire Water Tank Capacity Calculator with the design flow from this page.
Is this a final pump selection?
No. It is a preliminary size. The final pump must come from a listed pump curve and be approved by the authority having jurisdiction.