To size a water tank for a house in Nepal, multiply the number of people by the water each person uses in a day, then multiply by the number of days you want to store. A family of six using 100 litres per person per day needs 600 litres a day, so a 1,000 litre overhead tank covers one full day with some spare, and an underground tank of about 2,000 litres covers three days without supply. Tank volume in litres is length x breadth x water depth in metres x 1,000.
Most houses in Kathmandu, Pokhara and other towns need storage because the municipal or community supply comes for a few hours, on some days only, or not at all in the dry season. Buy too small and the family runs out; build too big and you pay for concrete, steel and roof load you do not need, and water sits long enough to go stale. This guide shows the numbers to use, a worked example, how to turn litres into tank dimensions, and what to check on site.
Step 1: How much water does one person use per day
Water demand is written in litres per capita per day (lpcd). There is no single figure that fits every Nepali home, so use one of these as a starting point and adjust for your family:
| Basis | Figure | When it fits |
|---|---|---|
| Basic rural service level used in Nepali water supply studies | 45 lpcd | Tap or yard connection, pour-flush toilet, careful use |
| Medium urban service level used in Nepali water supply studies | 100 lpcd | Town house with indoor taps, bathroom and washing |
| IS 1172 (Indian Standard, often referred to by designers in Nepal) | 135 lpcd | Modest city home with full flushing; about 45 litres of this is for flushing |
For a typical town house with a flush toilet, 100 lpcd is a sensible design value. Use 135 lpcd if the house has more than one bathroom, a washing machine or a garden that is watered from the same tank. If you have a tenant floor, count the tenants too. Your municipality, water user committee or design engineer may use a different local figure, so ask them before finalising the design.
Step 2: Daily demand by family size
Daily demand is simply people x lpcd. The table below is already worked out:
| People in the house | At 45 lpcd (litres/day) | At 100 lpcd (litres/day) | At 135 lpcd (litres/day) |
|---|---|---|---|
| 4 | 180 | 400 | 540 |
| 5 | 225 | 500 | 675 |
| 6 | 270 | 600 | 810 |
| 8 | 360 | 800 | 1,080 |
| 10 | 450 | 1,000 | 1,350 |
Step 3: Decide how many days to store
This depends on your supply, not on a code. Look at how often water actually comes to your house over a full year, including the dry months of Chaitra to Jestha when supply is usually worst.
- Overhead (roof) tank: usually sized for about one day of use. It feeds taps by gravity and is refilled by a pump from the underground tank or directly from the line.
- Underground tank: sized for the longest normal gap between supplies. If water comes every third day, store at least three days of demand.
- Tanker or boring water: if you buy water by tanker, the underground tank should hold at least one full tanker load so nothing is wasted. Ask local suppliers what size their tankers carry.
Many owners add 10 to 20 percent on top as a margin for guests and festivals. That margin is a judgement, not a rule.
Worked example: family of six in a town house
A family of six lives in a two storey house with one flush toilet. Municipal water comes every third day.
- Daily demand = 6 people x 100 lpcd = 600 litres per day.
- Overhead tank for one day = 600 litres. The next common plastic tank size above this is 1,000 litres, which also gives a buffer if the pump or power fails.
- Underground tank for three days = 600 x 3 = 1,800 litres, or 1.8 m³.
- Choose internal dimensions: 1.5 m long x 1.2 m wide x 1.2 m water depth = 2.16 m³ = 2,160 litres. That covers the 1,800 litres with about 20 percent spare.
- Add freeboard of about 0.3 m above the overflow level for the inlet pipe and air gap. Internal height becomes 1.5 m.
- In cubic feet, 2,160 litres is about 76.3 cft (1 cft = 28.32 litres), which helps if a mason or tank supplier quotes in cft.
Step 4: Turn litres into tank dimensions
1 cubic metre holds 1,000 litres. Work with internal dimensions, because the wall and floor thickness do not hold water.
- Rectangular tank: litres = L x B x H x 1,000, with H measured from the floor to the overflow pipe.
- Round tank: litres = 3.1416 x r x r x H x 1,000, where r is the internal radius. A round tank 1.2 m in diameter (r = 0.6 m) with 0.9 m water depth holds about 1,018 litres.
- Plastic tanks: the label gives nominal capacity. Usable water is a little less, because the outlet sits above the bottom and the overflow sits below the lid.
You can check any of these in seconds with the water tank calculator, which takes the shape and dimensions and gives litres. For conversions between m³, cft and litres, use the unit converter.
Overhead tank weight and where to place it
Water weighs 1,000 kg per cubic metre. A full 1,000 litre tank therefore puts about 1 tonne (roughly 10 kN) on the roof, plus the weight of the tank and stand. Two 1,000 litre tanks side by side put about 2 tonnes on a small area.
- Place the tank stand over columns or main beams, not in the middle of a slab panel.
- If the house was not designed for a roof tank, ask a structural engineer to check the slab before you add one, especially on an older load bearing house.
- Keep the tank base high enough to give pressure to the top floor taps. Each metre of height gives about 0.1 bar.
Site checks for an RCC underground tank
- Keep the tank well away from the septic tank and soak pit. Ask your municipality for the minimum distance in your area, as building bylaws differ.
- Use dense concrete, with proper cover to the bars and good vibration. Cure the walls and floor well before filling; see the guide on curing of concrete.
- Fill the tank and watch the water level for a day or two before plastering the outside or backfilling, so any leak shows up early.
- Give it a lockable manhole cover with a raised upstand, so surface water and dirt cannot run in.
- Slope the floor slightly towards a sump so you can empty and clean it once or twice a year.
To estimate the concrete for the walls, base and cover slab, work out each part with the concrete calculator.
Frequently asked questions
What size water tank does a family of five need?
At 100 lpcd a family of five uses about 500 litres a day. A 1,000 litre overhead tank is a common choice. Size the underground tank for the longest usual gap between supplies, for example 1,500 litres for three days.
How many litres are in a 1 m x 1 m x 1 m tank?
1,000 litres, measured inside the walls and up to the overflow level.
Is a bigger tank always better?
No. Water that sits for many days can go stale, and a larger tank costs more to build and clean. Store enough for your real supply gaps plus a small margin.
How do I convert tank litres to cubic feet?
Divide litres by 28.32. A 2,000 litre tank is about 70.6 cft.
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