Quick answer: Size a water storage tank from three numbers — how many people it serves, how much water each person uses a day, and how many days of storage you want. The core formula is storage (litres) = people × daily use per person × storage days. For a home, allow about 135 litres per person per day and roughly one day of storage, then split it between an underground sump and an overhead tank.
What decides tank size
A storage tank is a buffer between an unreliable supply and a steady demand. It has to hold enough water to ride through the hours — or days — when the mains is off, without running dry at peak use. Three inputs set the size:
- Population — the number of people the tank serves, or the equivalent for an office, school or hospital.
- Per-capita demand — how much water one person uses in a day, in litres per capita per day (LPCD).
- Storage period — how many days of supply you want to keep in reserve.
The water tank sizing formula
Total storage (litres) = population × per-capita demand (LPCD) × storage days
Tank volume (m³) = total storage ÷ 1000The result is the total water to store. That total is then divided between an underground tank (fed from the mains and any tanker) and an overhead tank (which feeds the taps by gravity). A common split keeps most of the water underground and a smaller working volume up top.
Enter people, demand and storage days — get the total litres and the underground and overhead tank split.
Open the Water Storage Tank Calculator →How much water does one person use?
Per-capita demand depends on the type of building. The figures below follow the National Building Code of India and are widely used across the region; other codes give similar numbers.
| Building type | Demand |
|---|---|
| Residential (with full plumbing) | 135 litres per person / day |
| Residential (low-income / EWS) | 70–100 litres per person / day |
| Offices | 45 litres per person / day |
| Factories (excluding process) | 30–45 litres per person / day |
| Schools (day) | 45 litres per student / day |
| Hostels | 135 litres per person / day |
| Hospitals | 340–450 litres per bed / day |
| Hotels | 180 litres per bed / day |
| Cinemas, assembly | 15 litres per seat / day |
The 135 LPCD residential figure already covers drinking, cooking, bathing, washing and flushing. If the supply also serves a garden or a car wash, add that separately.
How many people? Estimating population
When the exact number of occupants is unknown, estimate from the number of dwellings or bedrooms. A common planning rule is two people per bedroom, or an average of four to five people per flat for typical housing.
| Home | Assumed people | Daily demand at 135 LPCD |
|---|---|---|
| 1 BHK flat | 2–3 | 270–405 L/day |
| 2 BHK flat | 4 | 540 L/day |
| 3 BHK flat | 5–6 | 675–810 L/day |
| Independent house (family of 5) | 5 | 675 L/day |
How the storage is split
Total storage is rarely kept in one place. It is divided so the building stays supplied even when the mains is off and the pump is idle:
- Underground tank (sump). Holds the larger share — typically one full day of demand or more, plus any fire reserve. It receives water directly from the mains, which often arrives at low pressure and cannot reach an overhead tank on its own.
- Overhead tank. Holds a smaller working volume, commonly one-third to one-half of the daily demand. A pump lifts water from the sump to the overhead tank, which then feeds the taps by gravity.
A frequent arrangement is a sump sized for one to two days and an overhead tank sized for about half a day. The overhead tank should hold at least the peak morning and evening draw so the pump is not forced to run constantly.
Turning litres into tank dimensions
Once you know the volume, a rectangular tank is sized from length, breadth and depth. One cubic metre holds 1,000 litres.
Volume (litres) = length (m) × breadth (m) × water depth (m) × 1000Always add a freeboard of 150–200 mm above the water line so the tank does not overflow, and keep the depth practical — too shallow wastes floor area, too deep is hard to build and clean.
| Capacity | Example inside size (L × B × depth) |
|---|---|
| 500 L | 1.0 × 0.7 × 0.75 m |
| 1,000 L | 1.2 × 1.0 × 0.9 m |
| 2,000 L | 1.5 × 1.2 × 1.2 m |
| 5,000 L | 2.0 × 1.7 × 1.5 m |
| 10,000 L | 2.5 × 2.2 × 1.85 m |
Example — a 20-flat apartment block
A building has twenty 2 BHK flats. Estimating four people per flat gives 80 residents.
- Daily demand: 80 × 135 = 10,800 litres/day.
- Underground sump at one day: 10,800 L, rounded to a built 11,000 L tank — more if a fire reserve is required.
- Overhead tank at half a day: 5,400 L, rounded up to 6,000 L.
- Sump dimensions at 1.8 m depth: about 3.5 m × 1.8 m × 1.8 m water depth, plus freeboard.
So the block needs roughly an 11,000 L sump and a 6,000 L overhead tank, with a transfer pump sized to fill the overhead tank within the supply window. Size that pump with the booster pump sizing calculator.
Examples for other building types
The same formula works everywhere — only the per-capita figure changes. Three quick examples:
- Office for 100 staff. 100 × 45 = 4,500 L/day. A sump of about 5,000 L and an overhead tank of 2,000–2,500 L suit a single-shift office.
- Day school with 500 students. 500 × 45 = 22,500 L/day. Demand is concentrated in school hours, so keep a generous overhead tank of around 10,000 L to cover the mid-day peak.
- 50-bed hospital. 50 × 400 = 20,000 L/day for wards alone, before laundry and kitchens. Hospitals also need extra reserve for reliability, often two full days of storage.
Why storage duration is more than one day for many buildings
One day of storage suits a home on a fairly reliable supply. Where the mains is intermittent, or where running dry is unacceptable — hospitals, hotels, data centres — the storage period is stretched to two days or more. The storage period is a judgement about supply reliability and the cost of an interruption, not a fixed rule. On a supply that comes only a few hours a day, size the underground sump to take a full day’s water in that short window, because it must refill fast enough to last until the next supply.
Peak demand and the overhead tank
The overhead tank is not just a fraction of the daily total; it also has to cover the busy morning and evening periods when many taps run at once. If the transfer pump cannot keep up during those peaks, the overhead tank drains and pressure drops at the top floors. Sizing the overhead tank for at least the peak few hours of use, and setting the pump to refill it steadily, keeps supply even without the pump short-cycling.
Choosing the tank material
| Type | Where it fits |
|---|---|
| Moulded polyethylene | Overhead tanks up to a few thousand litres; light, cheap, quick to install |
| Reinforced concrete (RCC) | Large underground sumps and big overhead tanks; durable, built to any size |
| Ferrocement | Medium underground tanks; economical where cast on site |
| Sectional or pressed steel / GRP | Large roof tanks and plant rooms; bolted panels for big volumes |
Underground sumps are almost always cast concrete because of the size and the ground load. Overhead tanks up to a few thousand litres are usually moulded plastic; above that, concrete or sectional steel.
Do not forget the fire reserve
For larger and taller buildings, codes require a separate static fire-water reserve held in the underground tank on top of the domestic storage. This reserve is never drawn for daily use. The quantity depends on the building height and occupancy and is set by the local fire code, so treat it as an addition to the domestic figure, not part of it. For fire storage, see the fire water tank calculator.
Practical notes
- Round up to standard sizes. Moulded tanks come in fixed capacities — 500, 1,000, 2,000, 5,000 L — so choose the next size above your figure.
- Keep water fresh. Oversizing an overhead tank means water sits too long and can go stale; match it to real daily use.
- Allow for growth. If occupancy may rise, add a modest margin rather than sizing exactly to today’s headcount.
- Plan cleaning access. Every tank needs an inspection cover, a drain point and enough internal height to clean.
- Separate drinking storage. Where mains quality is poor, some homes keep a smaller, dedicated drinking-water tank.
Key points to remember
- Storage (litres) = people × per-capita demand × storage days.
- Allow about 135 LPCD for homes; less for offices and schools.
- Split the total: most underground, a smaller working volume overhead.
- One cubic metre holds 1,000 litres; add 150–200 mm freeboard.
- Add any code-required fire reserve on top of the domestic storage.
For the plumbing that follows, see NBC plumbing fixture requirements and the water supply pipe calculator.
Frequently asked questions
How do you calculate water tank capacity?
Multiply the number of people by the per-capita demand in litres per day and by the storage days. Divide by 1000 to get cubic metres.
What size water tank for a family of 5?
About 675 litres per day at 135 LPCD. A 1000 litre overhead tank with an underground sump holding a full day is a typical arrangement.
How much water storage per person?
Around 135 litres per person per day for homes with full plumbing, and less for offices, schools and factories.
How is storage split between sump and overhead tank?
Most of the water is held underground (a day or more, plus any fire reserve) and a smaller working volume, a third to a half of the daily demand, sits in the overhead tank.
How many litres are in a cubic metre tank?
1000 litres per cubic metre. Volume in litres equals length x breadth x water depth in metres, times 1000.