Fire water tank volume looks like a single multiplication: flow × duration. On site, the problems come from outside that multiplication: the wrong flow (density × area used instead of the hydraulic result), a forgotten hose and hydrant allowance, dead volume the pump cannot draw, a freeze allowance, and the question of how long the tank takes to refill once it is empty. This guide builds tank volume from start to finish, with examples on both the NFPA and EN 12845 side.
For a quick calculation use the fire water tank sizing calculator. For the tank's construction and fittings, NFPA 22 fire water tank design and, on the EN side, EN 12845 storage tank volume and refilling go deeper.
The basic equation
Effective tank volume is the total flow of every water user expected to operate at the same time, multiplied by the duration the standard requires:
Veffective = (Qsprinkler + Qhose/hydrant + Qother) × t
The geometric (gross) volume stated in the project and on the tank order adds dead volume, freeze allowance and freeboard on top:
Vgeometric = Veffective + Vdead + Vfreeze + Vfreeboard
Each term has its own source of error; we take them in turn below.
1. Flow: the hydraulic result, not density × area
Density × area is only the theoretical minimum sprinkler flow. Real demand is typically 10–25% higher because of the minimum pressure condition at sprinklers in the most remote area, node balancing and imbalance in the pipework. Tank volume should be calculated with the flow the hydraulic calculation gives at the system inlet. Where no calculation exists yet, adding 15–20% to the theoretical value is a reasonable start for preliminary sizing, but the final design must be updated with the hydraulic result. For the calculation steps, see the hydraulic calculation guide.
Other users to add to the sprinklers:
- Hose allowance (NFPA 13): LH 380 L/min, OH 950 L/min, EH 1900 L/min; for ESFR and storage criteria, the value in the table.
- Hose reels and hydrants: added where fed from the same tank and expected to run at the same time as the sprinklers. In Türkiye the national fire regulation's hose reel and hydrant provisions also apply (Turkish regulation on fire pumps and water tanks).
- Special systems: water curtains, drenchers, foam-water or deluge zones are added where the scenario counts them as simultaneous.
Simultaneity is an engineering decision: the demands of two separate fire scenarios are not added, but the demands of systems operating in the same scenario are. The report should show which scenario governs the tank.
2. Duration
| Standard / class | Duration | Note |
|---|---|---|
| NFPA 13 — LH | 30 min | |
| NFPA 13 — OH1 / OH2 | 60–90 min | Lower value where alarm and supervisory signals go to a constantly attended location |
| NFPA 13 — EH1 / EH2 | 90–120 min | Same condition |
| NFPA 13 — ESFR | 60 min | Hose allowance 950 L/min |
| NFPA 13 — other storage criteria | Per table | Varies with commodity and height |
| EN 12845 — LH | 30 min | |
| EN 12845 — OH1–OH4 | 60 min | |
| EN 12845 — HHP / HHS | 90 min |
Getting the hazard class right is a precondition for this table; for NFPA see the NFPA 13 hazard classification guide, and for EN the EN 12845 hazard class finder.
3. Effective volume and dead volume
A pump cannot draw the water at the bottom of the tank. As the water level approaches the suction inlet, a vortex forms, the pump draws air and discharge pressure collapses. The standards therefore define effective volume as the volume between the normal water level and the lowest level the pump can still draw safely.
- Height of the suction inlet above the floor: the inlet is kept a set height above the floor so that bottom sludge is not drawn in. Everything below it is dead volume.
- Submergence: a minimum depth of water must remain above the inlet to prevent vortexing. An anti-vortex plate reduces that depth but does not remove it.
- Suction sump: dropping the suction inlet into a sump below the tank floor cuts dead volume markedly and can be economical on large tanks.
The practical effect is large: on a tank with an 80 m² base, a 35 cm dead water column is 28 m³. Ordering a "200 m³" tank and ending up with 170 m³ effective is one of the most common mismatches on site. Always specify effective volume on the order, and put the suction detail on the tank drawing.
4. Freeze protection
- NFPA 22: requires heating so that tank water does not fall below 5.6 °C (42 °F); heater capacity is chosen from the lowest one-day mean temperature for the location and the tank surface area. A low water temperature alarm is advisable.
- EN 12845: for tanks where ice may form, raises the normal water level enough to allow for ice thickness (of the order of 1 m) and calls for ice venting; that extra volume does not count as effective. Confirm the value in the current edition.
- In practice: insulation, heating and circulation should be considered together. Suction and fill pipes, the level gauge and the overflow must also be protected from freezing.
For heater sizing in detail see FM Global DS 3-2 tank heating and freeze protection and EN 12845 frost protection strategies.
5. Refill time
After a fire or an accidental draw-down, how long will the tank take to refill? The site is unprotected for that whole period.
- EN 12845: a full-capacity tank must be capable of being refilled within 36 hours.
- NFPA 22: expects a much shorter fill time (of the order of 8 hours); confirm with the current edition and the authority.
For example, refilling 200 m³ effective volume in 36 hours needs ~5.6 m³/h (≈ 1.5 L/s); in 8 hours it needs 25 m³/h (≈ 7 L/s). The second figure may be beyond a small mains connection; the fill pipe size and mains pressure must be verified in the project.
6. EN 12845 reduced-capacity tanks
Where there is automatic infill from a reliable town main, EN 12845 allows tank volume to be reduced by the water the main will deliver during the fire:
Vtank ≥ max( Vmin ; Vfull − Qinfill × t )
The conditions are strict: infill only from the town main and automatic, through more than one mechanical float valve (failure of one must not reduce the required infill rate), with an inlet that does not disturb the suction, an infill rate that can be measured and verified, and a tank that never falls below the minimum volume tabulated by hazard class. The difference between "there is a main" and "there is a guaranteed main" is the crux: mains pressure and flow must be considered at the time of a fire, together with other demand in the area. For detail see EN 12845 reduced-capacity tanks.
7. NFPA 22 tank types
| Type | Strength | Watch point |
|---|---|---|
| Welded steel | Large volumes, long life | Internal coating and cathodic protection; site weld quality |
| Bolted steel (including factory-coated) | Fast, modular erection | Gasket and bolt sealing; coating damage |
| Reinforced / prestressed concrete | Economical for buried tanks or tanks built into the structure | Internal waterproofing, crack control, access for internal inspection |
| Glass-fibre reinforced plastic (GRP/FRP) | Corrosion-resistant, suits buried installation | Volume limits, loading and buoyancy forces |
| Elevated gravity tank | Pressure source without a pump | Structural and seismic design, freezing |
| Pressure tank | Small systems, pumpless supply | Limited volume; air/water ratio and pressure monitoring |
Whatever the type, the design should show: an externally readable level indicator, a vent (sized to prevent vacuum), an overflow, a drain, a manhole for internal inspection, an anti-vortex plate and a low level alarm. The FM Global DS 3-2 water tanks article is a useful comparison.
Worked examples
Example 1 — NFPA 13, OH2 production hall
- Sprinkler flow from the hydraulic calculation: 1300 L/min (theoretical 8.1 × 139 ≈ 1126 L/min).
- Hose allowance: 950 L/min. Total: 2250 L/min.
- Duration: supervision not to a constantly attended point → 90 min.
- Effective volume: 2250 × 90 = 202.5 m³.
- Tank base 8 × 10 m = 80 m²; dead water column 0.35 m → 28 m³.
- Freeboard 0.25 m → 20 m³.
- Geometric volume ≈ 202.5 + 28 + 20 ≈ 251 m³; effective water depth ≈ 2.53 m.
Example 2 — EN 12845, OH3 non-storage area
- Highest demand on a fully hydraulically calculated system: 1350 L/min; duration 60 min → full capacity 81 m³.
- Had the same area been designed as pre-calculated, the tabulated volume (which depends on height) would have been markedly larger; full hydraulic calculation can therefore save a great deal of tank volume.
- Reduced capacity: verified mains infill 700 L/min → 42 m³ in 60 minutes. 81 − 42 = 39 m³; but if the tabulated minimum for OH3 is higher, the minimum applies.
- Refill: for the 81 m³ full capacity, ≈ 2.3 m³/h over 36 hours.
Example 3 — ESFR warehouse
K360 ESFR, 12 sprinklers, assumed 2.4 bar: ~7000 L/min at the area inlet + 950 L/min hose = 7950 L/min × 60 min ≈ 477 m³ effective volume. Adding dead volume and freeze allowance puts the geometric volume in the 520–560 m³ band. The background to that calculation is in the ESFR sprinkler design guide.
One tank, one point of failure: when the tank is drained for cleaning or coating repair, the site has no water. A two-compartment tank (each compartment with its own suction and valve) or a second water supply keeps protection in place during maintenance. In tanks shared with domestic water, the domestic draw-off must sit above the fire reserve so that the reserve can never be used up.
Tank sizing checklist
- Is the sprinkler flow the value at the system inlet from the hydraulic calculation?
- Have hose, hose reel, hydrant and special system demands been added according to the simultaneity scenario?
- Is the duration right for the class and the supervision arrangement?
- Are effective and geometric volumes shown separately; is dead volume taken from the suction detail?
- Where there is a freeze risk, have heating, insulation and (for EN) a raised water level been added?
- Does the refill time meet the standard; have the fill pipe size and mains pressure been verified?
- If reduced capacity is used, are all conditions and the minimum volume met?
- Do the drawings show the level indicator, low level alarm, vent, overflow, drain and manhole?
- Has it been worked out how protection continues during maintenance?
To see how an inadequate water supply shows up on site, read inadequate water supply cases, and for the pump side see the fire pump room design checklist.
Frequently Asked Questions
How is fire water tank volume calculated?
Effective volume = (sprinkler flow from the hydraulic calculation + hose/hydrant and other simultaneous demands) × the duration the standard requires. For geometric volume, add dead volume, freeze allowance and freeboard.
What is the difference between effective and geometric volume?
Effective volume lies between the normal water level and the lowest level the pump can draw without vortexing. Water below the suction inlet and any freeze allowance do not count as effective.
How quickly must a tank refill under EN 12845?
A full-capacity tank must be capable of refilling within 36 hours. NFPA 22 expects a much shorter fill time; confirm it in the current edition.
Can the tank be smaller if there is a mains infill?
EN 12845 permits a reduced-capacity tank with automatic, verifiable mains infill, more than one float valve and a minimum volume by hazard class. On the NFPA side the main is assessed as a separate water supply.
What minimum water temperature must a tank hold?
NFPA 22 requires tank water not to fall below 5.6 °C (42 °F). Heater capacity is chosen from the lowest one-day mean temperature for the location and the tank surface area.
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Download MEP Calc on the App StoreNFPA 13 (2022/2025) · NFPA 22, Standard for Water Tanks for Private Fire Protection · BS EN 12845:2015+A1:2019 · FM Global DS 3-2 · Turkish Regulation on Fire Protection of Buildings. Flows and pressures in the examples are assumptions; duration, refill and minimum volume values vary between editions, and the binding text is the current standard and the authority having jurisdiction.