Fire Water Tank: How Many Cubic Meters You Need (NFPA 22)

The volume of a fire water tank is calculated by multiplying the system's design flow rate (sprinklers plus hose/hydrants, in liters per minute) by the required duration for the hazard (in minutes), then converting to cubic meters. The formula is V = Q × t. As a guide, light hazards call for on the order of 30 to 60 minutes of autonomy and high hazards up to 90 to 120 minutes or more. The exact volume is set by the hydraulic design of the system and the applicable code, typically NFPA 22 (tanks) together with NFPA 13 (sprinklers). Always verify the current standard and the local code with your engineer.

The method: flow times duration

The fire reserve is not a percentage of daily consumption: it is the volume that guarantees full-flow supply to the system for the entire fire event. It is calculated as:

V (m³) = Q (L/min) × t (min) ÷ 1,000

Both terms come from the fire protection design:

• Design flow (Q): the hydraulic demand of the system at the most remote point. It sums the sprinkler flow of the design area plus an allowance for hose streams/hydrants. It is set by the hydraulic calculation per NFPA 13 and the hazard classification of the occupancy.
• Duration (t): the minimum time the system must sustain that flow. It depends on the hazard category: the higher the fire load, the longer the required duration.

NFPA 22 governs the tank itself (materials, effective capacity, connections, level, protection). The flow and duration come from the system the tank feeds.

Reference durations by hazard

Autonomy times depend on the hazard classification. As a conceptual guide, automatic sprinkler systems work with these orders of magnitude (verify the exact value against the current standard and local code):

• Light hazard: offices, schools, hospitals. Duration on the order of 30 minutes.
• Ordinary hazard: medium manufacturing plants, moderate storage. Duration on the order of 60 to 90 minutes.
• Extra / high hazard (high-piled storage): logistics, flammables, tall warehouses. Duration of 90 to 120 minutes or more.

These are conceptual reference values: the real duration and flow come from the specific hydraulic calculation and the current edition of the standard.

Worked example: ordinary-hazard industrial building

1. Sprinkler flow: the hydraulic calculation of the design area gives 2,400 L/min at the most remote point.
2. Hose/hydrant allowance: add 950 L/min for interior and exterior hydrants.
3. Total design flow: Q = 2,400 + 950 = 3,350 L/min.
4. Required duration: for the project's ordinary hazard, adopt t = 90 minutes.
5. Volume: V = 3,350 × 90 ÷ 1,000 = 301.5 m³.
6. Adoption: size a tank of 320 m³ geometric, leaving margin and freeboard above the 301.5 m³ of usable fire volume.

If the same tank also supplies sanitary or process consumption, the fire volume is reserved as a dormant volume at the bottom (via a raised outlet or a partition), so daily use never depletes it. We develop this reserve logic in how to size a water storage tank.

Reference table: indicative fire volume

Fire reserve volume by design flow and duration. Indicative values for preliminary design; the real flow and duration are set by the hydraulic calculation and the current standard.

Hazard	Flow Q (L/min)	Duration t (min)	Volume (m³)
Light	1,500	30	45
Light-medium	2,000	60	120
Ordinary	3,000	60	180
Ordinary	3,350	90	302
Extra / high	5,000	90	450
Extra / high	6,000	120	720
High-piled storage	9,000	120	1,080

What else to check on the fire tank

• Effective capacity: NFPA 22 defines the volume actually usable between the suction level and the overflow; the geometric volume must exceed the usable fire reserve.
• Always-available reserve: if the tank is shared, the fire water must be physically separated from consumption (raised outlet, partition or dedicated tank).
• Material and coating: glass-fused-to-steel bolted tanks resist stagnant fire water without corrosion; the bolted steel tank erects fast and is sized to order.
• Connections and pumping: fire pump suction, fill line, overflow, vent, level indicator and access per the standard.
• Standard and local code: the final volume is validated against the current edition of NFPA 22 and NFPA 13 and against the local fire and permitting code.

If you need to define the flow, duration and volume of your fire water tank, Tech Tank sizes and supplies the tank turn-key. See our fire protection system solutions.

Frequently Asked Questions

How do you calculate the volume of a fire water tank?

With the formula V = Q × t: the system's design flow (sprinklers plus hose/hydrants, in L/min) times the required duration (in minutes), divided by 1,000 to get m³. The flow comes from the hydraulic calculation and the duration from the hazard classification.

What does NFPA 22 say about the volume?

NFPA 22 is the standard for water tanks for private fire protection: it defines materials, effective capacity, connections, level and fittings. The flow and duration that determine the volume come from the system the tank feeds (NFPA 13 for sprinklers) and the hazard classification. Always verify the current edition and the local code.

How much autonomy time should the system have?

It depends on the hazard. As a guide: light hazard on the order of 30 minutes, ordinary hazard 60 to 90 minutes, and extra hazard or high-piled storage 90 to 120 minutes or more. The exact value is set by the current standard according to the occupancy.

Can the same tank serve both fire and consumption?

Yes, as long as the fire volume is physically reserved and dormant: this is achieved with a raised consumption outlet or an internal partition, so daily use never consumes the fire reserve. It is a very common configuration in industry.

What material is best for a fire water tank?

Bolted steel with a glass-fused-to-steel coating resists stagnant water without internal corrosion, erects fast and is sized to the required volume. It is a common option versus concrete or field-welded steel.

What regulations apply in Paraguay?

System sizing relies on the NFPA standards (NFPA 22 for the tank, NFPA 13 for sprinklers) and the local fire and permitting code; always verify current regulations. The structural design of the bolted steel tank follows the AWWA D103 standard.