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Water Tank Size Calculator

Size your overhead tank from household demand, using the IS 1172 figures.

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Quick Answer

IS 1172:1993 sets the minimum water supply by town size: 100 to 150 litres per head per day in a town of 20,000 to 1 lakh with flush toilets, 150 to 200 in a bigger city, and it lets a city plan on 135 for LIG and EWS houses. This calculator starts at 135 litres. At that figure a family of four uses 540 L a day, so a 750 L tank holds one day. At the big-city 150 L minimum it is 600 L a day, still a 750 L tank. Where supply is irregular or you rely on a borewell, store 2 to 3 days: 1,500 to 2,000 L. Enter your household below to get the litres and the tank size to buy.

Figures checked Oct 2026. Source: IS 1172:1993 Code of Basic Requirements for Water Supply, Drainage and Sanitation (Bureau of Indian Standards): cl. 4.1 litres per head per day by town size and the 135 L note for LIG and EWS houses, cl. 4.1.1 45 L for flushing (printed pp. 1 and 2).

135 Lper person per day, our default (IS 1172)
540 La day for a family of four
45 Lof a city home's 150 to 200 L goes on flushing
750 Ltank for one day, family of four

Part of the system: Water tank and sump size. Steps, checks and a free printable page.

Guide: Tank and sump litres on a quote.

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The Formula

Tank size = people × litres per person per day × days of storage, rounded up to a standard tank size

The litres per person per day is an input you can see and change, not a hidden constant. IS 1172 gives a range by town size, so pick the line that matches your town and plumbing and the whole working updates.

IS 1172 minimum water supply, litres per head per dayTown under 20,000, no flush toilets: 70 to 100 L. Town of 20,000 to 1 lakh with flush toilets: 100 to 150 L. City over 1 lakh with flush toilets: 150 to 200 L. Line at 135 L.Town under 20,000, no flush toilets70 to 100 LTown of 20,000 to 1 lakh, flush toilets100 to 150 LCity over 1 lakh, flush toilets150 to 200 L135 L default
IS 1172 minimum water supply, litres per head per day. The top of each range, by town size. The line marks the 135 L this calculator starts at.

Assumptions

  • Demand figures from IS 1172:1993 cl. 4.1: 70 to 100 litres per head per day for a town under 20,000 with a house connection and no flush toilets; 100 to 150 for a town of 20,000 to 1 lakh with full flushing; 150 to 200 for a city over 1 lakh with full flushing, which the code lets you cut to 135 for LIG and EWS houses. Of the 150 to 200 litres, 45 go on flushing (cl. 4.1.1). NBC 2005 Part 9 cl. 4.1.1 repeats the same figures.
  • The result is domestic storage only. Garden, car washing, construction curing and firefighting storage are extra
  • Suggested sizes follow the standard rotomoulded tank ladder: 500, 750, 1000, 1500, 2000, 3000, 5000 L and up
  • Where a sump + overhead tank combination is used, the total across both should meet the storage figure; the overhead tank alone can then be smaller

Worked Examples

Family of four, by days of storage1 day 540 L, tank 750 L; 2 days 1,080 L, tank 1,500 L; 3 days 1,620 L, tank 2,000 L.1 day: 750 L tank540 L2 days: 1,500 L tank1,080 L3 days: 2,000 L tank1,620 L
Family of four, by days of storage. 135 L a person, rounded up to the next standard tank size.

Family of 4, municipal supply, 1 day of storage

  1. Daily demand: 4 people × 135 L/person/day = 540 L/day
  2. Storage: 540 L/day × 1 day = 540 L
  3. Next standard tank size: 750 L

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Family of 6 on borewell, 2 days of storage

  1. Daily demand: 6 people × 135 L/person/day = 810 L/day
  2. Storage: 810 L/day × 2 days = 1,620 L
  3. Next standard tank size: 2,000 L

Load this example in the calculator

Tools on Amazon.in

For measuring and running the tank

Tools to measure the space, plus two common ways to stop overflow.

A pump controller connects to mains power. Hire a licensed electrician for any wiring work.

  • Measures the tank space and works out volume.MiLESEEY 70 m laser distance meter (4.5★, 9,400 ratings) · See on Amazon.in
  • For the smaller measurements.Stanley 5 m measuring tape (4.5★, 27,100 ratings) · See on Amazon.in
  • Switches the pump on and off by tank level.Microtail automatic water level controller (4.2★, 3,000 ratings) · See on Amazon.in
  • Shuts the inlet when the tank is full. It works without any wiring.New Ware PVC float valve (4.1★, 318 ratings) · See on Amazon.in

Or search Amazon.in yourself. Type what you need, or tap a common search.

water level controllerfloat valve for tanklaser distance metermeasuring tape

As an Amazon Associate we earn from qualifying purchases. Ratings checked October 2026. We don't show prices; Amazon has the current one.

How to Use This Calculator

  1. Count everyone. Include live-in help and long-stay guests. The norm is per person, and undercounting is the usual reason tanks run dry.
  2. Pick the usage figure. Use 135 L for a town house or a modest city flat, 150 L or more for a city house with a flush toilet and a shower in every bathroom, 90 L where there are no flush toilets.
  3. Decide the storage days. Reliable daily supply needs 1 day. Alternate-day municipal supply, summer cuts or borewell dependence justify 2 to 3 days.
  4. Buy the suggested size. Tanks come in standard sizes, so the calculator rounds up for you. Slightly over is fine; grossly over means stagnant water and a heavier roof load.

Tips and tricks

  • Size the tank for the busiest month, not the average. Summer bathing, guests over Diwali and a wedding in the house all push demand up for weeks.
  • A 1,000 L tank is 1 m³ of water and weighs about a tonne full. Put the stand over a beam or a wall, not the middle of the slab.
  • Flush toilets take 45 L of a city home's 150 to 200 L a day (IS 1172 cl. 4.1.1). If you plumb flushing from a separate tank, size that one at 45 L a head.
  • With a sump below and a tank on the roof, the two together carry the days. NBC 2005 Part 9 cl. 4.4.10 puts half a day on the roof where supply is intermittent, so the roof tank can stay small.

What we would do

For a family of four in a city, we'd use 150 L a head, which is IS 1172's floor for a big city with flush toilets. That gives 600 L a day. On daily supply a 750 L roof tank is enough. On alternate-day supply we'd add a sump of at least 1,200 L and keep the roof tank at 750 L.

Frequently Asked Questions

What tank size does a family of 4 need?
About 540 L a day at 135 litres a head, or 600 L at the big-city minimum of 150. Either way a 750 L overhead tank covers daily municipal supply; take 1,000 to 1,500 L where supply is irregular. With an underground sump doing the main storage, a 500 to 750 L roof tank plus a sump of 1,000 L or more is the usual split.
Why does the calculator start at 135 litres per person per day?
135 is the figure IS 1172:1993 allows for LIG and EWS houses in a city, and it sits inside the 100 to 150 L range for a town of 20,000 to 1 lakh with flush toilets. The code's minimum for a city over 1 lakh with full flushing is 150 to 200 L, with 45 L of that for flushing. If you live in a big city and every bathroom has a flush toilet and a shower, pick 150 or 180.
Should I use an overhead tank, an underground sump, or both?
Both, in most Indian cities: the sump collects whenever supply arrives (municipal timing is rarely convenient) and holds 1 to 2 days of demand; a pump lifts to a smaller overhead tank (typically half a day to a day) that gravity-feeds the house. Single overhead tanks suit places with reliable pressure and daily supply.
How big is a 1,000 litre tank physically?
1,000 L is exactly 1 m³. A typical rotomoulded 1,000 L tank is roughly 1.1 m in diameter and 1.1 to 1.2 m tall, and weighs about 1,000 kg when full. That is why tank platforms on roofs need to sit over beams or walls, not mid-slab. Always check the slab or frame can take the full weight.
Does the calculation cover gardening, car washing or construction?
No. IS 1172 counts household use only. A garden or regular car washing needs its own allowance, so pick a higher figure per person or add the litres separately, rather than let outdoor use eat the household's buffer.
Is a bigger tank always better?
No. Water stored beyond about 3 days can stagnate and lose residual chlorine, oversized rooftop tanks impose serious structural loads, and dead storage is money spent to make water quality worse. Size to genuine need plus a sensible buffer; the days-of-storage input is the setting to change.
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Sources & Limitations

What this calculator does not model

  • Sizes domestic storage only. Firefighting storage, rainwater harvesting and irrigation are separate calculations
  • Does not size the pump, pipes or float valves; it sizes the tank
  • Apartment buildings follow their own municipal and NBC provisions. This is for individual homes
  • Structural adequacy of the tank platform must be checked by an engineer for anything above ~1,000 L on a roof