Storage = occupants × litres per person per day × days + fire reserve. 40 people at 135 L/day for one day plus 10,000 L fire reserve is 15,400 L. Split ⅓ / ⅔, that is a 5.1 m³ overhead tank (about 1.84 m square, 1.5 m deep) and a 10.3 m³ sump; filling the overhead tank in an hour needs about 85 L/min.
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Daily = 40 × 135 L = 5,400 L; total = daily × 1.0 + 10,000 fire = 15,400 L
Overhead = 33% = 5,082 L; sump = 10,318 L
Square tank side = √(volume ÷ water depth); add freeboard (typically 150–300 mm) to the internal height
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Formulas
Daily demand = occupants × L per person per dayStorage = daily × days + fire reserveOverhead = storage × overhead share; sump = the restSquare tank side = √(volume ÷ water depth)
Worked Example
40 occupants, 135 L/day, 1 day, 10,000 L fire reserve, ⅓ overhead at 1.5 m depth, sump at 2.0 m:
Daily = 5,400 L; storage = 15,400 L
Overhead = 5,082 L → ≈ 1.84 × 1.84 m; sump = 10,318 L → ≈ 2.27 × 2.27 m
Pump to fill the overhead tank in 1 h ≈ 85 L/min
Tank Capacity by Size
Full capacity to the stated water depth. Usable volume is less once freeboard is left at the top.
Tank (L × W × depth)
Cubic feet
Litres
US gallons
4 × 4 × 4 ft
64 ft³
1,812 L
479 gal
5 × 5 × 5 ft
125 ft³
3,540 L
935 gal
6 × 6 × 5 ft
180 ft³
5,097 L
1,346 gal
8 × 6 × 5 ft
240 ft³
6,796 L
1,795 gal
10 × 8 × 6 ft
480 ft³
13,592 L
3,591 gal
How to Size a Building Water Tank
Count the people the building serves.
Pick the daily use per person from your local code (135 L in IS 1172 for homes).
Multiply by the days of storage you want.
Add any fire reserve the fire authority requires.
Split the total between the overhead tank (often a third) and the underground sump.
Choose a water depth and work out the tank sides, then add freeboard.
Frequently Asked Questions
How much water does a person use per day?
Planning figures vary, about 135–200 litres per person per day in homes and flats (IS 1172 suggests 135), around 45 litres per person in offices and day schools. Use your local code.
How big should an overhead water tank be?
Often about a third of the total storage, with the rest in an underground sump that a pump lifts from. Add fire reserve as the fire authority requires.
How much freeboard does a tank need?
Commonly 150–300 mm above the top water level, for the overflow and air gap.
How do I calculate water tank capacity in litres?
Multiply length × width × water depth. In metres, the answer × 1,000 is litres: 2 × 1.5 × 1.2 m = 3.6 m³ = 3,600 L. In feet, multiply the cubic feet by 28.32: a 5 × 5 × 5 ft tank holds 125 ft³, about 3,540 L.
How do I calculate the capacity of a round tank?
Volume = π × radius² × water depth. A tank 2 m across and 1.5 m deep holds 3.1416 × 1² × 1.5 = 4.71 m³, which is 4,712 L.
