Underground Water Tank Size: How to Calculate It

A 2,000-litre tank for a family of five sounds generous. For a single day without municipal water it is more than enough. For the three-day supply failure that most Indian neighbourhoods see at least once a year — a burst main, a pump-house breakdown, a festival-week shortage — it is barely two days of normal use, and that is before a single bucket goes to the garden or the car. The size of an underground water tank is not a number to copy from the neighbour's plan; it is a calculation, and the calculation is short enough to do on the back of a bill.

The mistake runs the other way too. Many bungalow owners, frightened by one bad summer, pour a 20,000-litre underground tank that never fills past a quarter, sits with stale water in the bottom, and costs a small fortune to build and clean. An oversized tank is not a safety margin; it is a standing pool of money and, if the water turns, a health risk. The right size sits between these two errors, and it is found the same way for a 1BHK flat and an eight-bedroom farmhouse: estimate how much water the household actually uses in a day, decide how many days of that use you want to hold in reserve, and build a tank that holds it with a little room to spare.

This article sets out the method a designer uses, with the per-capita figures from Indian water-supply practice, worked examples for common household sizes, the internal dimensions that turn a litre figure into a buildable box, where the tank should and should not go, how the underground sump works with the overhead tank, and what the whole job costs in 2026. The structural design of a water-retaining RCC tank should always be confirmed by a licensed structural engineer and a plumber; this guide is for getting the size and the brief right before they arrive.

The method: demand, days and a margin

Every tank size comes from one line of arithmetic. Daily demand is the number of people in the house multiplied by how much water each person uses in a day. Storage is that daily demand multiplied by the number of days you want to hold in reserve against a supply failure. Add a small margin, round up to a buildable size, and that is the tank. The only judgement calls are the per-capita figure and the number of days — and both have sensible defaults drawn from Indian practice.

For the per-capita figure, Indian water-supply practice commonly takes about 135 litres per person per day for a home with full indoor plumbing and a flush toilet, a figure long used in the Bureau of Indian Standards code IS 1172 and in the Central Public Health and Environmental Engineering Organisation (CPHEEO) manual. Homes without full plumbing use less, around 70–100 litres; large homes with gardens, cars, multiple bathrooms and RO systems that waste reject water use more, often 150–200 litres. Metros frequently plan on 150 litres or more. The 135-litre figure is a safe starting point for a typical urban flat or a modest bungalow; adjust it up for a garden and a lifestyle that uses water freely.

For the number of days, one day of storage is the bare minimum and only works where the supply is reliable and comes daily. Where supply is intermittent — an hour in the morning, nothing on some days, the occasional tanker — two to three days of reserve is sensible, and that reserve lives mostly in the underground sump. A household on a borewell with a good yield can hold less; a household that depends on tankers should size the underground tank to swallow a full tanker load, usually several thousand litres, in one delivery. The table below breaks the 135-litre figure into where the water actually goes, which also shows where a household can cut demand if the tank must stay small.

Worked tank sizes for common households

With the method and the per-capita figure in hand, the tank size for any household is a short sum. The table below runs it for the household sizes most Indian homes fall into, at 135 litres per person per day (and a higher 160-litre figure for the bungalow with a garden). Read it as a guide to the order of magnitude, not a precise prescription: the last column, the suggested underground sump, is the practical reserve a designer would build, rounded up to a sensible size and allowing for a tanker delivery where relevant.

Two things about this table are easy to misread. First, the underground sump column is larger than the two-day figure in some rows because it rounds up to a standard buildable size and, for larger homes, allows the sump to receive a tanker in one go rather than in two trips. Second, the sump does not have to hold the whole reserve on its own — it works with the overhead tank, which is covered below. A 3BHK with a 2,500-litre sump and a 1,000-litre overhead tank together hold about three-and-a-half thousand litres, comfortably more than three days for five people, which is why those are the sizes most Surat and Ahmedabad flats are built with.

From litres to a buildable box

A capacity in litres has to become a length, a breadth and a depth before anyone can build it. The conversion is simple: one cubic metre of water is exactly 1,000 litres, so a tank measuring one metre in each direction holds 1,000 litres when full to the brim. In practice a tank is never filled to the brim — a freeboard of about 150–300 millimetres is left above the top water level so the tank does not slop over and so the inlet and overflow have room to work. Depth is kept to about two to two-and-a-half metres at most, because a deeper tank is harder to excavate, to waterproof and, above all, to climb into and clean. The table below gives workable internal dimensions for the common sizes.

Those are internal clear dimensions and gross volumes rounded down for freeboard — a 1,500 mm cube holds about 3,375 litres gross but roughly 3,000 litres usable once a 150 mm freeboard is kept. The walls and base add to the dig: an RCC wall is typically 150–230 millimetres thick and the base raft similar, so an excavation is always larger than the clear tank. The section below shows the details that matter as much as the size — the freeboard, the manhole, the vent, and the outlet kept clear of the silt that always settles on the floor.

Underground sump and overhead tank: who does what

Most Indian homes run a two-tank system, and sizing one without the other leads to trouble. The underground sump is the reserve and the receiver: municipal supply and tanker water fill it, and it holds the bulk of the storage because it is cheap to build large and easy to hide below the yard. But water cannot reach the taps from below ground without pressure, so a pump lifts it to the overhead tank. The overhead tank is the working store: it holds roughly a day's use, gives the house gravity pressure, and empties and refills every day, which keeps the water fresh. Splitting storage this way means the sump can be big for security while the overhead tank stays small enough to sit on the terrace without overloading the slab.

Where to put it, and the rules that protect the water

An underground tank holds water a family drinks and cooks with, so its position is a health decision, not only a convenience one. Keep the tank well clear of the septic tank, the soak pit and any drainage line — a common rule of thumb is a separation of at least 7.5 metres, and more in sandy or made-up soil where contamination travels further; follow the local health norms where they are stricter. Never place a drinking-water tank directly below a toilet or a kitchen waste line. The tank must be a sealed box: a tight, lockable manhole cover keeps out leaves, lizards and runoff, and a vent pipe with a mesh lets air move without letting insects in. The inlet should discharge above the top water level so dirty water can never siphon back into the mains.

Structurally, a water-retaining tank is not an ordinary box. It is designed to hold water without cracking or leaking, which is why the Bureau of Indian Standards has a separate code, IS 3370, for liquid-retaining concrete structures, and why the concrete grade, the wall thickness and the reinforcement should come from a structural engineer rather than a thumb rule — especially for a large tank, a tank under a driveway that carries vehicle loads, or a tank in high water-table ground, where an empty tank can float. Waterproofing is built in from both faces: a dense, well-cured concrete, often with an integral or crystalline waterproofing admixture, plus a smooth internal plaster with a cove at every corner so there are no sharp joints to crack. A ponding test — filling the finished tank and watching the level for a few days before covering it — is the simplest proof that it holds.

What it costs to build in 2026

An underground RCC tank is priced by the job, but it helps to think of it per litre of storage, because that is how the size drives the cost. The ranges below are indicative October 2026 figures that include excavation, the RCC raft and walls, waterproofing and internal plaster, but exclude the pump, the plumbing connections and any extra for difficult soil, a high water table or a slab strong enough to park on. Confirm every figure with a local contractor and a structural engineer before budgeting, as rates vary with city, soil and access.

Two costs are worth planning for even if the tank is small. The first is cleaning: budget a yearly desilting and disinfection, which is cheap and keeps the water safe. The second is the reserve you did not build — if the first summer shows the tank runs dry, extending an underground tank later means breaking open the yard, so it is far cheaper to build one size up at the start than to add capacity afterwards. Aaditya Interior plans the sump, the pump and the overhead tank together at the drawing stage, so the slab, the plumbing shafts and the terrace loads are all ready for the sizes the household will actually need.

Rules for sizing and building an underground tank

• Start from people, not from a round number: daily demand is persons multiplied by about 135 litres (more with a garden).
• Decide the reserve honestly — one day only where supply is reliable, two to three where it is not.
• Let the underground sump hold the reserve and the overhead tank hold about a day, so daily water stays fresh.
• Size the sump to receive a full tanker in one delivery if the home depends on tankers.
• Convert litres to a box at 1,000 litres per cubic metre, and leave 150–300 mm of freeboard above the top water level.
• Keep the depth to about 2–2.5 metres so the tank can be cleaned, and build in a 600 × 600 mm manhole and a vent.
• Place the outlet about 150 mm above the floor and slope the floor to a desilting pit so silt is never drawn into the house.
• Keep the tank at least 7.5 metres from the septic tank and soak pit, and never below a toilet or waste line.
• Have a structural engineer design any large tank, any tank under a driveway, and any tank in high water-table soil.
• Fill and ponding-test the finished tank before covering it, and clean and disinfect it at least once a year.

Frequently asked questions

How do I calculate the water tank size for my house?

Multiply the number of people by their daily water use — about 135 litres per person per day for a fully plumbed home — to get the daily demand, then multiply that by the number of days you want to hold in reserve. A family of five needs about 675 litres a day, so two days is roughly 1,350 litres and three days about 2,025 litres. Round up to a buildable size and split it between an underground sump and an overhead tank.

What is the standard underground water tank size for a 3BHK home?

There is no single standard, but a 3BHK of about five people is commonly built with an underground sump of 2,000–3,000 litres and an overhead tank of around 1,000 litres, which together give roughly three days of reserve. Homes that depend on tankers or have a garden go larger, often a 3,000–5,000 litre sump.

How many litres is a 1 metre by 1 metre tank?

A tank with internal dimensions of one metre in each direction holds one cubic metre, which is exactly 1,000 litres when full to the brim. In practice you leave a freeboard of 150–300 millimetres above the top water level, so the usable volume of a one-metre cube is closer to 800–850 litres.

Should the underground tank be bigger than the overhead tank?

Usually, yes. The underground sump is cheaper to build large and holds the main reserve against supply failures, while the overhead tank is kept to about a day's use so it refills daily and the water stays fresh, and so its weight does not overload the terrace slab. A big sump and a modest overhead tank is the normal arrangement.

How far should the water tank be from the septic tank?

Keep a drinking-water tank well away from the septic tank and soak pit — at least about 7.5 metres is a common rule, and more in sandy or made-up soil where contamination spreads further. Never place the tank directly below a toilet or a waste line, and follow the local municipal health norms where they are stricter.