How do you size a fire water tank?
Tank capacity is the largest simultaneous water demand sustained for the duration the standard requires. For sprinkler systems the demand has two parts: sprinkler discharge and, where applicable, a hose or hydrant allowance. Total flow multiplied by duration gives the effective volume; the unusable volume below the suction inlet is then added on top.
V = (Qsprinkler + Qhose) × t ÷ (1 − unusable share)Sprinkler demand
Sprinkler demand is design density times area of operation. A handy identity: mm/min × m² = L/min (and 1 gpm/ft² = 40.75 mm/min). In a hydraulic calculation the remote-area sprinklers run above minimum pressure, so the real flow is higher than the simple product; the overrun factor represents that gap. Once the hydraulic calculation exists, replace it with the calculated flow.
| NFPA 13 class | Density | Area | Hose | Duration |
|---|---|---|---|---|
| Light | 0.10 gpm/ft² | 1500 ft² | 100 gpm | 30 min |
| Ordinary 1 | 0.15 gpm/ft² | 1500 ft² | 250 gpm | 60–90 min |
| Ordinary 2 | 0.20 gpm/ft² | 1500 ft² | 250 gpm | 60–90 min |
| Extra 1 | 0.30 gpm/ft² | 2500 ft² | 500 gpm | 90–120 min |
| Extra 2 | 0.40 gpm/ft² | 2500 ft² | 500 gpm | 90–120 min |
The values above are the largest-area points of the NFPA 13 density/area curves. For EN 12845 the calculator uses the Table 3 design points (LH 2.25 mm/min over 84 m², OH1–OH4 5 mm/min over 72–360 m², HHP1–HHP3 7.5–12.5 mm/min over 260 m²) with 30, 60 and 90 minute durations.
In dry-pipe and alternate systems water reaches the open heads later, so more sprinklers open: EN 12845 increases the area of operation by 25%, NFPA 13 by 30%.
Unusable volume
Not all of a tank is usable. Water below the suction inlet, the minimum submergence above an anti-vortex plate and the freeboard below the overflow sit outside the effective volume. Ten percent is a sensible preliminary allowance; replace it with the real geometry once the suction detail is fixed.
Common mistake: buying a tank by its nominal capacity and counting it as effective capacity. A tank sold as 300 m³ can leave 25–30 m³ of unusable water below the suction.
Refill time
EN 12845 requires a full-capacity tank to be refillable within 36 hours; NFPA 22 practice commonly targets 8 hours. The calculator shows the fill flow implied by the selected standard. If the town main cannot deliver it, you need a larger tank or a dedicated make-up source.
When is this calculator not enough?
- For storage hazards (HHS, rack storage, ESFR) demand depends on commodity class, storage height and in-rack sprinklers — use the hydraulic result directly as the total flow.
- Where foam, water curtain or deluge systems operate simultaneously, combine the largest simultaneous demand.
- EN 12845 reduced-capacity tanks rely on a secure continuous inflow and follow a different sizing route.
Frequently Asked Questions
How big should a fire water tank be?
It depends on the hazard class. For EN 12845 OH3 wet: 216 m² × 5 mm/min × 1.15 overrun ≈ 1242 L/min, which over 60 minutes is ≈ 75 m³ effective; with a 10% unusable allowance the gross tank is about 83 m³.
Is the hose allowance added to tank capacity?
Under NFPA 13, yes: 100 gpm for light, 250 gpm for ordinary and 500 gpm for extra hazard are added to the sprinkler demand. EN 12845 adds none, but include hydrant or hose-reel water required by local rules if it comes from the same tank.
Why does a dry system need a bigger tank?
Water reaches the open sprinklers later and the fire grows meanwhile, so more heads open. The area of operation increases by 25% in EN 12845 and 30% in NFPA 13, and demand rises by the same ratio.
How do I choose the tank diameter?
Pick the effective water height from site limits and pump suction conditions; the diameter then follows from V = π·D²·H/4. Squat tanks with a diameter-to-height ratio above 4 need a large footprint and a more expensive base.
SprinkCalc — Fire Sprinkler Design in Three Standards
Run these calculations on site, offline and with PDF reports: NFPA 13, FM Global and BS EN 12845 in one iOS app.
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