The typical scenario maybe like this. 

So I had so much space under my parking lot. Can I build a giant water tank in here? So that I don’t need to worry about the storage issue anytime.

Big No! 

Water Tank Capacity Requirement

Be it underground or overhead water tanks the capacity calculation is essential.

If you build a tank with huge capacity than the required quantity, you may not need to worry about the storage issue, but you will waste a lot of money in other ways.

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What happens if you build a huge tank than your actual requirement?

  • Every time there will be some water leftover. Storing water for a long time will make it contaminated now the old water contamination will get mixed with the new one. There are also health hazards.
  • Second thing, it needs to be maintained regularly. With a big tank, It becomes a headache for you. If it is underground, then you need to hire manpower often to clean it properly.

To avoid such conditions you need to calculate your average daily requirement along with your scheduled water refill. According to that, you can build an optimum water tank size with economic benefits.

Capacity Calculation Formula

According to IS Code 1172 (1993), the value of the minimum water requirement has been retained as 135 litres per head per day. It varies based on the building types. 

Example Calculation

Assume we need to calculate the size of a water tank for a residential building with 10 persons.

Total Water Requirement = Number of persons X Minimum Water requirement = 10 X 135 Litres = 1350 Litres

We know, 1 m3 = 1000 litres of water.

So the required storage volume for 1350 Litres = 1.35 Cubic Metre.

Now we have calculated the volume of water requirement.

Overhead or Underground Water Tank? – It is up to you. Normally an overhead tank requires more strong support to build which affects your budget. Underground tanks, on the other hand, do not need any kind of strong support since we place it on the ground.

Underground Water Tank Size Calculation

Assuming we are going to construct a rectangular water tank with depth as 1 metre.

Underground Water Tank

We know volume formula, Volume = Area X Depth

Area of the tank = 1.35 / Depth = 1.35/1 = 1.35 m2 or square metre

Area of rectangular = Length X Breadth (Assuming L=2B)

Therefore, 1.35 m2 = 2B X B

1.35 = 2B2

2B2 = 1.35

B2 = 1.35/2 = 0.675

B = √0.675 = 0.82 m

So length = 2 X B = 2 X 0.82 = 1.64 metre.

Add an additional 300mm for the free flow


Bala is a Planning Engineer & he is the author and editor of Civil Planets.


  1. Hema Priya Reply

    It’s a good experience to know the technical aspectsand extrordinary things.


    For the 50000liter what might be the area I need to for the construction of underground Tank


    Thank you for help me .I need to have you assistance because I am a student in civil engineering but I haven’t monney for paying you.

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