Fire Water Tank Capacity Calculator (Sprinkler and Hydrant)
How the formula works
A fire water tank must store enough water to run the fire protection system for the required time without any other supply. The size depends on the flow that the system needs and on how long it must run. This calculator finds the tank capacity for a sprinkler system, a hydrant (standpipe) system, or both. It can also check whether an existing tank is big enough.
Formulas
Sprinkler flow = Design density × Design area
Total flow = Sprinkler flow + Hose stream allowance + Hydrant flow
Storage = Total flow × Duration
Tank capacity = (Storage − Make-up flow × Duration) × (1 + Safety margin)
Available duration of an existing tank = Tank capacity ÷ (Net flow × (1 + Safety margin))
Where
- Design density = water applied per unit area, in L/min per m² (or GPM per ft²)
- Design area = the area over which the sprinklers are assumed to operate
- Hose stream allowance = extra flow for fire fighters’ hoses, added for sprinkler only systems
- Hydrant flow = flow demand of the hydrant or standpipe system
- Duration = the time the water must last, in minutes
- Make-up flow = a reliable water supply that refills the tank during the fire, only if the code allows it
Hazard classes and design values (NFPA 13)
- Light hazard: density 0.10 GPM/ft² over 1,500 ft², hose allowance 100 GPM, duration 30 minutes
- Ordinary hazard 1: density 0.15 GPM/ft² over 1,500 ft², hose allowance 250 GPM, duration 60 minutes
- Ordinary hazard 2: density 0.20 GPM/ft² over 1,500 ft², hose allowance 250 GPM, duration 90 minutes
- Extra hazard 1: density 0.30 GPM/ft² over 2,500 ft², hose allowance 500 GPM, duration 90 minutes
- Extra hazard 2: density 0.40 GPM/ft² over 2,500 ft², hose allowance 500 GPM, duration 120 minutes
These are typical values. NFPA 13 gives a range of durations for some classes, so you can select a duration yourself. Your local code, the insurer and the fire authority can ask for other values. Choose Custom if you have project specific values.
Hydrant (standpipe) flow
NFPA 14 commonly asks for 500 GPM for the first standpipe and 250 GPM for each additional one, up to 1,250 GPM. Other codes use different values, so choose Custom for your own flow.
Unit conversions
1 GPM = 3.785 L/min
1 GPM per ft² = 40.75 L/min per m²
1 m³ = 1,000 litres = 264.17 US gallons
How to use the calculator
- Select the calculation and the fire protection system.
- For a sprinkler system, select the hazard class. For a hydrant system, select the hydrant flow.
- Select the duration (use the standard value for the hazard class, or pick minutes), and the safety margin.
- Enter a make-up supply only if your code allows a reduction. Enter the water depth if you want the tank plan size.
- To check an existing tank, enter its capacity in litres.
- Click Calculate. You can then download the result as a PDF.
Important notes
- Domestic water and fire water are often stored separately, or the fire water is kept in a dedicated section of the tank. Follow your project specification.
- The suggested fire pump rating is the next standard NFPA 20 size above the total flow. The pump must also be selected for the required pressure.
- The result is a planning estimate. The final design must be approved by the fire authority.
Worked example
Example 1: Sprinkler only system (light hazard)
An office building has a sprinkler system only. The hazard class is light hazard, the duration is the standard 30 minutes, and the safety margin is 10 %. The water depth in the tank is 3 m.
Step 1: Sprinkler flow
0.10 GPM/ft² × 1,500 ft² = 150 GPM = 568 L/min
Step 2: Hose allowance
100 GPM = 379 L/min
Step 3: Total flow
568 + 379 = 946 L/min (250 GPM)
Step 4: Storage
946 × 30 = 28,391 L (28.39 m³)
With 10 % margin: 28,391 × 1.10 = 31,230 L (31.23 m³), about 8,250 US gallons
Step 5: Tank size and pump
Square plan side = √(31.23 ÷ 3) = 3.23 m, so the tank is 3.23 m × 3.23 m × 3 m water depth
Suggested fire pump: 250 GPM (946 L/min)
Example 2: Sprinkler and hydrant system
A shop building has ordinary hazard 2 sprinklers and a hydrant system with a flow demand of 750 GPM. The duration is 90 minutes and the safety margin is 10 %.
Sprinkler flow = 0.20 × 1,500 = 300 GPM = 1,136 L/min
Hydrant flow = 750 GPM = 2,839 L/min
Total flow = 3,975 L/min (1,050 GPM, 238 m³/h)
Storage = 3,975 × 90 = 357,716 L
With 10 % margin = 393,488 L (393.49 m³), about 103,948 US gallons
Suggested fire pump: 1,250 GPM (4,732 L/min)
Example 3: Check an existing tank
A building has ordinary hazard 1 sprinklers only (60 minutes). A reliable make-up supply of 500 L/min refills the tank. The safety margin is 10 %. The existing tank holds 60,000 L.
Sprinkler flow = 0.15 × 1,500 = 225 GPM = 852 L/min
Hose allowance = 250 GPM = 946 L/min
Total flow = 1,798 L/min (475 GPM)
Storage = 1,798 × 60 = 107,884 L
Less make-up supply = 500 × 60 = 30,000 L, so net storage = 77,884 L
With 10 % margin = 85,673 L
The existing tank (60,000 L) is short by 25,673 L. It can run for about 42 minutes against the 60 minutes required. The suggested fire pump is 500 GPM (1,893 L/min).
Common mistakes
- Using the wrong hazard class. The class depends on the fuel load and how the space is used. A higher class needs much more water.
- Forgetting the hose stream allowance. A sprinkler only system still needs water for the fire fighters’ hoses.
- Adding the hose allowance twice. When a hydrant system is present, its flow takes the place of the separate hose allowance.
- Using a short duration. Ordinary and extra hazard areas need 60 to 120 minutes, not 30 minutes. Check the code.
- Counting the make-up supply without approval. Many authorities do not allow the tank to be reduced for an incoming supply unless it is very reliable.
- Mixing fire water with domestic water. If the fire reserve is part of a shared tank, the domestic outlet must be placed so that the fire water cannot be used up.
- Ignoring the safety margin. Losses, unusable water at the tank bottom and later changes in the occupancy can reduce the usable water.
- Mixing GPM and L/min. 1 GPM is 3.785 L/min. Check the unit of the pump and the code table.
- Not checking the pump pressure. The tank and the pump flow are only part of the design. The pump must also give the pressure at the most remote point.
Frequently asked questions
How do I calculate the fire water tank capacity?
Find the total flow of the system (sprinkler flow plus the hose allowance or the hydrant flow) and multiply it by the required duration. For example, 946 L/min for 30 minutes is about 28,391 litres.
How is the sprinkler flow calculated?
Multiply the design density by the design area. For light hazard, 0.10 GPM/ft² over 1,500 ft² gives 150 GPM, which is about 568 L/min.
What is the hose stream allowance?
It is the extra water flow that is assumed to be used by fire fighters’ hoses at the same time as the sprinklers. NFPA 13 gives 100 GPM for light hazard, 250 GPM for ordinary hazard and 500 GPM for extra hazard.
How long must the fire water last?
Typical durations are 30 minutes for light hazard, 60 to 90 minutes for ordinary hazard and 90 to 120 minutes for extra hazard. Your local code and fire authority decide the final value.
What is the difference between a sprinkler system and a hydrant system?
A sprinkler system puts out or controls a fire automatically at the point of the fire. A hydrant (standpipe) system supplies water to hoses that fire fighters use by hand.
How much water does a hydrant system need?
NFPA 14 commonly asks for 500 GPM for the first standpipe and 250 GPM for each additional one, up to 1,250 GPM. Other codes, such as the NBC of India and local rules, can give different values.
Can I add the domestic water to the fire water tank?
Only if the design keeps the fire water protected from the domestic use. Often the fire water is stored in a separate tank or in a dedicated section.
Can I reduce the tank size when there is a make-up supply?
Some codes allow it if the supply is very reliable and can give the flow during the fire. Many do not. Enter a make-up value only if your fire authority approves it.
How do I choose the fire pump rating?
The pump rating is the next standard size above the total flow. The calculator uses the NFPA 20 standard ratings, such as 250, 500, 750, 1,000 and 1,250 GPM. The pump must also be selected for the pressure that the system needs.
How do I check if my existing fire tank is enough?
Select the check option and enter the tank capacity in litres. The calculator shows the surplus or shortfall and how many minutes the tank can supply the fire flow.
Does this replace a fire protection design?
No. It is a planning aid. The final design must follow the applicable code and be approved by the fire authority.