Water tank and sump size for a family: litres for the roof tank and the sump
Most houses in India get water for a few hours a day, or every other day, so the sump and the roof tank have to carry the family until the next supply. Size both from the people in the house and the gap between supplies, then write the litres and the inside sizes into the drawing.
By Bulan Sarkar · Updated
Multiply the people in the house by 135 litres a day: a family of five uses 675 litres a day. Put half a day on the roof, which is the National Building Code's minimum for an overhead tank where supply is intermittent: 338 litres, so a 500 litre tank. Size the sump for the gap between supplies: 675 litres for daily supply, 1,350 litres for alternate-day supply, 2,700 litres if water comes only every four days. For one day's storage the code's split is two-thirds in the sump and one-third on the roof, so never build the sump smaller than the roof tank.
The decision: size the tank and sump yourself?
Yes, for an individual house or a floor added to one. The litres are simple arithmetic, and this page takes you from the head count to the inside sizes to write on the drawing and the quote.
If the house already has a sump and a roof tank and only the plastic tank is old or cracked, skip the sizing. Buy the same net capacity, unless more people live there now than when it was built.
The sump's walls, floor, slab and waterproofing, and the beams or walls a roof tank sits on, are for a licensed engineer. Get a plumber for the pump, the float valve and the pipes. Apartment blocks and fire-fighting storage follow their own rules (NBC Part 4), so leave those to the building's engineer.
The steps, in order
Count the people who live in the house
Have ready:the names of everyone who sleeps in the houselive-in help and long-stay relatives
NBC Part 9 cl. 4.3.1.1 says to base a home's water on the actual number of people in it, and to take a family of five when you don't know yet, for example while the house is still on paper. Count live-in help and parents who stay for months at a time. If a floor will be rented out, count that household separately (see the variants).
Pick the litres per person per day
Have ready:how many bathrooms have flush toiletswhether you run a washing machine
NBC Part 9 cl. 4.1.1 sets minimums by town size. A town of 20,000 to 1 lakh people with full flushing gets 100 to 150 litres per head per day, and a city over 1 lakh gets 150 to 200, which the code lets you cut to 135 for LIG and EWS houses. Out of the 150 to 200 litres, 45 go on flushing (cl. 4.1.1.1).
Our water tank calculator starts at 135 litres, and this page uses 135 in its examples. If the house is in a big city and every bathroom has a flush toilet and a shower, plan on 180 instead.
Daily demand = people × litres. A family of five at 135 litres needs 675 litres a day.
Write down how often the water comes
Have ready:the supply timings, from the neighbours or the water boardthe longest gap in supply you remember from last summer
The code lists what decides the storage: the hours of supply at enough pressure to fill the tank, how often it comes in a day, how regular it is, and what happens when the stored water runs out (cl. 4.4.10). Turn that into one number: the days the house has to manage without fresh supply. Daily supply for a few hours is 1 day. Alternate-day supply is 2. Water twice a week, from the board or a tanker, is up to 4. Use the longest gap you actually saw last summer, not the timetable.
Size the overhead tank: half a day
Have ready:the daily demand from step 2the plastic tank sizes your dealer stocks
Where supply is intermittent and the hours are irregular, NBC cl. 4.4.10 asks for at least half a day's supply in the overhead tank. Half of 675 litres is 338. Plastic tanks come in 500, 750, 1,000 litres and up, so the family of five buys a 500 litre tank.
Keep the rest of the storage in the sump. A full 1,000 litre tank weighs about a tonne with its water, and the code wants tanks on a roof to rest on structural members that carry that load straight down (cl. 4.4.4 h). Your engineer marks where the stand goes. Raise it so the tank's outlet is at least 2 m above the highest tap or shower on the top floor (cl. 4.4.8); lower than that and the top-floor shower runs weak.
Size the sump for the gap between supplies
Have ready:the days to carry from step 3the calculator's storage figure for that many days
Sump litres = daily demand × days to carry. Put your people, litres and days into the water tank calculator and it gives this figure. For the family of five: 675 litres for daily supply, 1,350 for alternate-day supply, 2,700 for every four days.
We don't subtract the roof tank. When the supply comes back, the roof tank is usually half used, and the sump has to refill both. Check the answer against the code too. For one day's storage the NBC note to cl. 4.4.10 puts 66.6% in the underground tank and 33.4% overhead, which is 450 and 225 litres for this family, so 675 litres covers the code's split. If you buy tanker water, the sump must also take a whole tanker load in one go, so ask the supplier how many litres the tanker delivers.
Turn the litres into inside sizes, to the overflow
Have ready:a measuring tapethe plan of the plot with the sump marked
Litres = inside length × inside width × water depth, in metres, × 1,000. The water depth runs from the floor to the overflow, not to the slab. For 1,350 litres, a sump 1.5 × 1.0 m inside with 1.0 m of water to the overflow stores 1,500 litres.
Not all of it reaches the pump. The code puts the outlet 50 to 75 mm above the tank floor, with a strainer (cl. 4.4.6), and the water below it stays where it is. With the outlet 75 mm up, the pump can lift 1.5 × 1.0 × 0.925 = 1.39 m³, about 1,390 litres. That still covers the 1,350 litres. Our guide on checking sump and tank litres on a quote works through more examples.
Place the sump and mark its pipes on the drawing
Have ready:the site plan with the septic tank, soak pit and drains on itthe ground level at the gate and at the sump
NBC cl. 4.4.7 says an underground tank should not sit in a low-lying spot or near a sewer, septic tank, leaching pool or soak pit, and should share no wall with a tank holding water from an unsafe source. Its overflow should end well above the ground, preferably 600 mm, as a warning pipe with a mosquito-proof grating. Our guide on where to put the septic tank and soak pit on a small plot helps you keep them apart.
For both tanks, the overflow pipe is normally one size bigger than the inlet (cl. 4.4.4 c), and neither the overflow nor the scour pipe may connect to a drain, gully trap or manhole. They discharge over a grating with a 50 mm air gap (cl. 4.4.4 f). Write these on the drawing so the plumber sees them before the walls go up.
What goes wrong
Variants: what changes the steps
Water every day, for a few hours
Carry 1 day. The family of five builds a sump for 675 litres or more and puts a 500 litre tank on the roof. Supply that comes every day can still fail in summer, so if you remember long gaps, size for 2 days.
A tanker or the board, twice a week
Carry up to 4 days: 2,700 litres for five people, 3,240 for six. The sump has to take a full tanker in one go, so ask the supplier how many litres they deliver and build at least that much. The roof tank stays at half a day.
A floor rented out
Count each household on its own. Five people upstairs and four below is nine at 135 litres, 1,215 litres a day. Give each floor its own roof tank so one household can't empty the other's water. The shared sump carries the days for all nine.
Sump only, with a pressure pump
No tank on the roof: the pump pushes water straight to the taps. The NBC note to cl. 4.4.10 sizes an underground tank on its own at 50 to 150% of a day's demand. Where supply comes with gaps, size it for the days to carry instead, as in step 5. Every tap then depends on the pump and the power, so keep an inverter or a plan for cuts.
Tips and tricks
- Write down the longest gap in supply from last summer and size for that, not for the timetable the water board publishes.
- Ask the plumber to take the flushing outlet from a lower level than the other outlets. NBC cl. 4.4.11 suggests it, so the last of the water in the tank is kept for the toilets.
- Keep the roof tank at half a day and put the extra storage in the sump. The water stays cooler underground and the roof carries less weight.
- If the storage you need is over 5,000 litres, build two connected tanks so one can be shut off and cleaned while the other supplies the house (NBC cl. 4.4.5).
- Put the overflow pipe where someone will see it running. It is the warning that the float valve has failed and the pump is wasting water.
What we would do
For a family of five on alternate-day supply, we would build the sump 1.5 × 1.0 m inside with 1.0 m of water to the overflow, which stores 1,500 litres, on the side of the plot away from the septic tank and the soak pit. On the roof we would put a 500 litre tank on a stand the engineer places over a beam or wall, with its outlet at least 2 m above the top-floor shower. If the supply is daily and reliable, we would still build the sump for a full day rather than the code's two-thirds, because one bad summer week uses up the difference.
Print the working copy
Water tank and sump sizing sheet, from the Water Tank and Sump Planner. One A4 page with tick boxes and blanks; it prints in black and white.
- 1. People and litres
- 2. Supply
- 3. Roof tank
- 4. Sump
- 5. Before the walls go up
Frequently asked questions
What size water tank does a family of 4 need?
What should the ratio of sump to overhead tank be?
How big should a sump be for a house?
How high should the overhead tank be above the roof?
How far should the sump be from the septic tank?
The source we built this on
Read 21 Oct 2026 from the page images (Part 9 Section 1, printed pp. 18 to 22). NBC 2016 is the current edition; BIS sells it and its full text is not free to read, so if your engineer works from 2016, check any clause numbers here against it. The 135 litre default is our calculator's setting, and the family sizes, sump sizes and supply gaps are our worked examples, not figures from the code.
This page plans litres and sizes. The sump's walls, floor, slab and waterproofing, and the stand a roof tank sits on, are for a licensed engineer to design. Don't climb into a sump or tank to measure it; measure from the manhole or from the drawings.