Water Tank Capacity: engineer's guide
Domestic water storage covers demand between refills. A typical target is one day of consumption plus a safety margin, subject to civil defence and health authority minima.
Open the Water Tank Capacity →
What this calculator does
The calculator sums daily demand per occupancy and applies the reserve factor to return required tank volume, then converts to physical tank dimensions.
Formulas used
- Storage volume:
V = Qdaily × ndays × Ksafety
Variables
- Qdaily = total daily demand, L/day
- ndays = reserve duration, days
- Ksafety = 1.10 to 1.20 typical
Worked example
Scenario: 80-unit residential, 250 L/person/day, 4 persons/unit, one day reserve.
- Qdaily = 80 × 4 × 250 = 80,000 L
- V = 80,000 × 1 × 1.15 = 92,000 L
- Tank dimensions: 6 m × 5 m × 3.1 m usable depth (add 300 mm dead + 200 mm freeboard)
Common mistakes to avoid
- Undersizing without civil defence reserve
- Single-compartment tanks (no maintenance drawdown)
- Missing anti-vortex plate on outlet
- Ignoring cold-water storage temperature rise in hot climates
Frequently asked questions
GRP or steel?
GRP for potable water ≤ 60°C, coated steel for larger volumes.
Separate fire reserve?
Yes — fire water storage should be dedicated, not shared with domestic.
Standards and references
BS 6700, ASHRAE Applications Ch. 50, IPC 606
Related tools on this site
Open the calculator that pairs with this guide: 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.