How Do You Size a Fire Protection Water Storage Tank?

A fire protection water storage tank is sized by multiplying the largest water demand on the building, sprinklers plus the hose stream allowance, by the duration the sprinkler standard assigns to the hazard, then adding the water the tank holds but cannot deliver: the dead storage below the suction outlet and the air space above the overflow. For a typical ordinary hazard building the result lands between 30,000 and 90,000 gallons; storage buildings and standpipe buildings push it higher. The arithmetic takes 10 minutes. Getting the inputs right is the work.

This article is for owners and design teams on sites without a public main strong enough to carry the sprinkler demand: rural New Jersey, Pennsylvania, Maryland and Delaware properties on wells or small purveyors, and any site where the water authority will not commit to the flow. It covers the sizing method, a worked example with real numbers, the mistakes that produce undersized tanks, and where the simple method stops and a Professional Engineer (PE) calculation takes over. Whether a tank is needed at all is a separate question, covered in DGA’s article on when a fire protection water tank is required.

What Inputs Does a Tank Sizing Calculation Need?

Every number in the calculation comes from a document the design team already has or can request. The table lists each input, its units and where it comes from. The sprinkler demand is the one most often taken from the wrong place: the tank sees the demand at the base of the riser, or at the pump suction, not the density at the remote sprinkler.

Two of these inputs deserve a sentence each. The hose allowance is part of the stored volume because the fire department will draw it from the same tank through the hydrants on the private main; NFPA 13 (2022) Chapter 19 sizes a tank for the equipment it serves, so a tank that serves sprinklers only can leave the hose stream demand out. The refill rate matters because NFPA 22 expects the supply to put the required volume back within a fixed time, and a 20 gpm domestic well cannot do that for a 67,500 gallon demand.

What Is the Formula for Tank Volume?

The sizing method has five steps. Compute the sprinkler case, compute the standpipe case where one exists, take the larger, add the water the tank cannot deliver, and round up. Refill credit comes last and only where the standard and the AHJ allow it.

Figure 1: Tank Volume Formula

Sprinkler case: V_s = (Q_sprinkler + Q_hose) × t_s, where Q is in gpm at the base of the riser, t_s is the NFPA 13 duration in minutes and V_s is in gallons.

Standpipe case: V_p = Q_standpipe × 30 min for a Class I or Class III system under NFPA 14 Chapter 9.

Required effective capacity: V_eff = the larger of V_s and V_p in a building sprinklered throughout; the sum only where the AHJ requires both to run at once.

Gross tank volume: V_tank = V_eff + V_dead + V_air, where V_dead is the water below the lowest usable level and V_air is the space between the overflow and the roof.

The method follows the water supply chapter of NFPA 13 and the capacity rules of NFPA 22. NFPA 22 (2023) measures this capacity, which it calls net capacity, from the inlet of the overflow down to the lowest level the outlet can draw, which for a suction tank is the top of the vortex plate. Everything below that level is dead storage; everything above the overflow is air. Neither counts.

Figure 2: Sizing Steps in Order

1. Sprinkler caseBase of riser demand plus hose allowance, times the Chapter 19 or storage chapter duration.

2. Standpipe caseStandpipe demand from NFPA 14 Chapter 7, times the 30 minutes NFPA 14 Chapter 9 sets.

3. Governing demandThe larger of the two in a building sprinklered throughout, the sum only where the AHJ requires it.

4. Add what the tank cannot deliverDead storage below the vortex plate or outlet, and the air space above the overflow.

5. Round up and check refillChoose the next stock tank size, then confirm the source can refill the effective capacity within the NFPA 22 time.

Worked Example: A 75,000 Gallon Tank for an Ordinary Hazard Building

Take a 40,000 sq ft light manufacturing building in rural Pennsylvania on a well that tests at 60 gpm. The sprinkler system is designed as ordinary hazard group 2. The hydraulic calculation shows 500 gpm at the base of the riser, and NFPA 13 (2022) Chapter 19 adds 250 gpm of hose stream allowance and a 60 to 90 minute duration for ordinary hazard; the design uses 90 minutes because the waterflow alarm is not monitored at a constantly attended location. There are no standpipes.

The 10 percent allowance is a planning figure, not a code number. The actual dead storage comes from the tank drawing. In the figure below the tank is 28 ft 3 in in diameter and 16 ft tall, which holds 4,690 gallons per foot of depth. The vortex plate sits 1 ft above the floor, the overflow 14.5 ft above it, so 14.5 ft of water counts: 68,000 gallons of effective capacity against the 67,500 gallons required.

Figure 3: Where the 67,500 Gallons Sit in a 75,000 Gallon Tank

The refill check is where rural sites fail. NFPA 22 (2023) Chapter 4 requires the supply to refill the minimum required fire protection volume in no more than 8 hours, which needs 141 gpm here. A 60 gpm well refills 67,500 gallons in about 19 hours. The usual answer is a second well or a larger refill source; a slower refill or a tanker contract needs the AHJ’s acceptance as an alternative to that rule, and none of these reduce the stored volume.

Worked Example: When Standpipes and Sprinklers Compete

Now take a 4-story office building in Maryland with Class I standpipes and an ordinary hazard group 1 sprinkler system, on a small purveyor that cannot supply the fire demand. The building has three Class I standpipes, so the demand under NFPA 14 Chapter 7 is 500 gpm for the first plus 250 gpm for each additional, 1,000 gpm, which is also the maximum the standard requires in a building sprinklered throughout. The sprinkler calculation shows 500 gpm at the base of the riser, plus 250 gpm of hose, for 60 minutes, the lower ordinary hazard value NFPA 13 Chapter 19 permits where waterflow and supervisory signals are monitored at a constantly attended location.

The sprinkler case governs even though the standpipe flow is higher, because its duration is twice as long. The two demands are not added: NFPA 14 Chapter 7 lets the larger of the sprinkler and standpipe demands set the supply in a combined system where the building is sprinklered throughout. A design team that adds them buys an 85,000 gallon tank (75,000 gallons effective plus 10 percent, rounded up) for a 50,000 gallon problem.

The pump drawing from either tank is a separate design. Its rated flow and pressure come from the demand curve, not from the tank size, and DGA’s article on how to size a fire pump for rated flow and pressure covers that calculation. What the tank calculation owes the pump is the low water level, because NFPA 20 Chapter 4 requires the pump to deliver its rated flow with the tank at that level and with enough net positive suction head to avoid cavitation.

DGA sizes fire protection water tanks, fire pumps and private fire service mains across New York, New Jersey, Pennsylvania, Maryland and Delaware. Project briefs get confirmation and preliminary code feedback within one business day. See where DGA works →

Which Errors Undersize a Fire Protection Tank?

Most undersized tanks trace back to one of six input mistakes. Each one is easy to make from a summary sheet and easy to catch on the hydraulic calculation and the tank drawing.

Freezing deserves one more sentence for the rural audience. NFPA 22 (2023) treats tank heating as part of the tank, with the heater sized for the local low temperature and the water temperature and level supervised through the fire alarm system under NFPA 72. An unheated tank north of Maryland is an impaired water supply for three months of the year.

When Does Tank Sizing Need a PE Calculation?

The method above is reliable for a light or ordinary hazard building with one sprinkler system, with or without a standpipe, and a tank at grade. Beyond that the inputs stop being table lookups and the sizing becomes an engineering decision that a licensed Professional Engineer should seal.

Storage occupancies. A warehouse designed under NFPA 13 (2022) Chapters 20 to 25 takes its duration, hose allowance and demand from the storage chapter that fits the commodity, the storage height and the sprinkler type, not from Chapter 19. ESFR designs carry their own hose and duration values from Chapter 20, and the base of riser flow can exceed 2,000 gpm. Those tanks commonly run 100,000 gallons and up, and the choice of sprinkler type changes the tank by tens of thousands of gallons.

Combined and foam systems. A building with multiple sprinkler risers, a combined standpipe system with pressure-regulating valves, or a foam-water system has demands that do not fit a two-line comparison. Foam systems add a concentrate duration and a discharge rate that NFPA 11 governs, and the water behind the foam is stored at the same tank.

Pumps with suction limits. Where the tank feeds a fire pump, NFPA 20 Chapter 4 ties the lowest usable level to the pump’s net positive suction head and the anti-vortex plate submergence. A tall, narrow tank keeps the suction flooded; a wide, shallow tank may force the pump to be lowered or the dead storage to grow. The pump decision itself, including whether the site needs one at all, is explained in DGA’s guide to how engineers decide whether a fire pump is required.

Supply curves and insurer requirements. Where a weak main and a tank work together, the stored volume depends on how the two supplies combine over the duration, which is read from the plot described in DGA’s article on plotting sprinkler demand against the supply curve. FM Global, through its Data Sheet 3-2 on water tanks, generally expects the entire protection demand to be held in the tank and treats refill as a backup rather than a credit, so an FM-insured site should size to the insurer’s basis before the AHJ’s.

Who Approves the Tank in Each State?

The arithmetic is the same in every jurisdiction DGA works in outside New York City, where Appendix Q adds its own tank rules. What changes is who reviews the tank, which edition of NFPA 22 the adopted building code references, and which NFPA 13 edition supplies the duration. In Maryland, the county building permit reviews sprinklers to NFPA 13 (2019) through the 2021 International Building Code (IBC), while the State Fire Prevention Code (NFPA 1, 2024) references NFPA 13 (2022).

A county reviewer in Pennsylvania or Maryland reads the duration from NFPA 13 (2019) and a New Jersey reviewer from NFPA 13 (2022); the durations for light, ordinary and extra hazard are the same in both, so the tank does not change across the state line. The edition does matter for storage designs, where the chapter numbering and some hose values differ, and for Delaware, where the State Fire Marshal reviews to NFPA 13 (2025).

Frequently Asked Questions

This article is general information, not a substitute for a project-specific water supply analysis. Code editions, local amendments and reviewer practice change; confirm the adopted code and the authority having jurisdiction for the project.