A water tank that is too small runs dry by evening. One that is too large keeps water standing for days and adds unnecessary load on your roof slab. Sizing it correctly takes one simple calculation: how many people, how much water each uses, and how many days of storage you want.
How much water does a person use?
Indian guidance for water supply in cities commonly uses about 135 litres per person per day for homes with full flushing systems. Actual use varies widely: smaller towns, water-saving fixtures and careful habits use less, while gardens and washing cars use more.
Overhead tank vs sump
- Overhead tank: feeds taps by gravity. Usually sized for about a day's use so water stays fresh.
- Underground sump: stores water from the municipal supply or a tanker, then pumps it up. Often sized for one to two days, more where supply is irregular.
Worked example
A family of five at 135 litres each needs about 675 litres a day. A 1,000-litre overhead tank gives a comfortable margin, and a sump of around 1,500 to 2,000 litres covers supply gaps.
Structural and hygiene tips
- Load: a full 1,000-litre tank weighs about a tonne. Place it over beams or columns, not the middle of a slab, and check with your engineer.
- Clean tanks every six months and keep lids tightly closed against mosquitoes.
- Insulate or shade overhead tanks in hot climates so water is not scalding in summer.
- Float valves and overflow alarms save water and electricity.
Frequently asked questions
Plastic tank or concrete tank?
Plastic tanks are cheap, light and easy to replace. RCC tanks last longer and suit larger capacities, but are heavier and need waterproofing.
How high should the overhead tank be?
A few metres above the highest tap gives reasonable pressure. Showers and some geysers need more, which may require a small pressure pump.