How to Calculate Rectangular Water Tank Capacity and Size

How to Calculate Rectangular Water Tank Capacity and Size

Inlet and Outlet

The inlet and outlet allow the water to enter and exit the water of the tank for daily uses. Therefore water tank placement should be done at the right location. Water tanks are mostly installed outside of the house.  

Overflow Pipe

The overflow pipe is located at the inlet level at the top of the tank so that more water is not stored on the water tank. The overflow pipe is required for the freeboard to maintain the air gasp otherwise the tank might burst due to excessive pressure.

How to Calculate Rectangular Water Tank Capacity and Size 1


The drain is located at the bottom of the tank to remove the residual deposits material while cleaning the tank.


It is located at the top of the tank to release air on the water tank.vent is Air removed out through vent pipes at the time of refilling water in the tank.

Level Indicator

It is a dipstick or graduated rod that indicates the quantity of water stored inside the tank.


It is located at the base of the water tank for drainage purposes. The additional sump may be required for installation of submersible pumps in case of on groundwater tanks or underground.


Manhole is an opening into a water tank top for inspection, maintenance and cleaning. a manhole is an opening in the top of the water tank with a lid that can be locked.

How to Calculate Rectangular Water Tank Capacity and Size

Manhole is the basic components of a water storage tank that need to be checked at the time of installation properly.

The volume of Water Requirement

According to per IS code,  135 litres is needed for daily use per person per day. The breakup of the IS code assumptions.

How to Calculate Rectangular Water Tank Capacity and Size
  • Drinking – 5 litres
  • Cooking – 5 litres
  • Bathing & Toilet  – 85 litres
  • Washing Clothes & Utensils – 30 litres
  • Cleaning House – 10 litres

Capacity Calculation and Water Tank Size

For a typical family (8 members),

Total water requirement is 135 litres x 8 = 1080 litres per day.

We all know Volume of water formula as 1 cum = 1000 litres

In order to get the size of the water tank, you need to mention at least one dimension of Width or Depth, Length, of the water tank.

If you are planning to construct a water tank near a car parking area there may be some restriction on length.

That’s the reason why we are planning to construct the water tank underground while foundation work.

the volume of water formula, see that above,

1 m3 = 1000 litres

1 litre = 0.001 m3

Our requirement is 1080 litres,

Therefore, 1080 litres = 1.08 m3

Assuming our water tank depth as 0.8 m

Area of tank = 1.08/0.8

Area of tank = 1.35 m2

Assume the length of the water tank is 2 m.

Width of the water tank = 1.35/2

Width of the water tank  =0.675 m

Note: you can also replace value your input if you know the length

 So, For 1080 litre water tank size, L = 2 m, B = 0.675 m & D = 0.8 m

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