Fire Water Tank Capacity: engineer's guide
Fire water storage covers the worst-case simultaneous fire demand for the code-required duration.
Open the Fire Water Tank Capacity →
What this calculator does
The calculator sums sprinkler, hose, and hydrant demand, multiplies by the applicable duration, and returns usable and gross tank volume.
Formulas used
- Usable volume:
Vuse = (Qsprinkler + Qhose + Qhydrant) × tduration - Gross volume:
Vgross = Vuse + Vdead + Vfreeboard
Variables
- Q = flow rate in GPM or L/s
- tduration = 30 to 120 minutes per hazard class
- Vdead = last 300 mm below suction bell
- Vfreeboard = 150 to 300 mm above max water level
Worked example
Scenario: Ordinary hazard Group 2 building: sprinkler 650 GPM including hose, plus 500 GPM hydrant simultaneously, 90 min duration.
- Vuse = (650 + 500) × 90 = 103,500 gallons ≈ 392 m³
- Add 8% dead + 5% freeboard → Vgross ≈ 440 m³
- Tank: 12 m × 8 m × 5 m gross
Common mistakes to avoid
- Sizing on sprinkler demand only
- Assuming utility refill credit without AHJ approval
- Missing dead volume and freeboard
- Not providing two-compartment tank for maintenance
Frequently asked questions
Duration by hazard?
Light 30 min, Ordinary 60 to 90 min, Extra 90 to 120 min, storage per NFPA 13 Ch. 20.
Concrete or steel tank?
Concrete for underground, steel for above ground; both need internal coating for potable-adjacent use.
Standards and references
NFPA 22, NFPA 13, NFPA 20
Related tools on this site
Open the calculator that pairs with this guide: Fire Water Tank Capacity.
Verification note
This guide is planning support only. Final design must be verified against the applicable project specification, authority requirements, manufacturer data, and the standards cited above. Consult a licensed engineer for construction-issue calculations.