How to calculate gallons in a tank

     Hi Everyone, Welcome to "The Civil Engineer". In this article let me explain how to calculate a tank's capacity.

    Before going further, understand that if you have to say the capacity of a tank is 5000 liters, you need to design the tank with more than 5000 liters. The additional capacity is called the freeboard. The freeboard is given so that the sloshing waves hit and damage the tank's roof. We will come in a minute on how to calculate the freeboard.

How to Design/Calculate the Capacity of a Rectangular Water Tank:

    Let us take that we need to design a tank with a capacity of 5000 liters. On the site, we have this Length and breadth as 2.34m and 1.52m respectively. From this, we can calculate the height.

Multiply the Length and Breadth we get 3.556 sqm.

We all know that 1cum = 1000litres = 264.17gallons, hence if we convert the 5000litre capacity into volume we get, 5cum.

So now, 5/3.556 =  1.41m which is the height of the tank without a freeboard. As I said before freeboard is very important in designing the tank hence a minimum of 250mm to 300mm of freeboard needs to be given. let us take 300mm = 0.3m, so the height of the tank will become 1.71m.

So from the above calculation, the Inner dimension of the tank for a capacity of 5000 liters with freeboard needs to be 

    Length = 2.34m

    Breadth = 1.52m

    Depth = 1.41m(A) + 0.3(FB) = 1.71.

Hence the capacity of the tank along with the freeboard is 6.08cum = 6082 litres = 1605.6 Gallons.

How to Design/Calculate the Capacity of a Circular Water Tank:

Let us take we need to find the dimensions of a circular tank with a capacity of 5000 liters. Let us take there is enough space to make a circular tank with a diameter of 2.34m. Then we need to find the height of the tank.

Area of the tank(Inner Dimension)  = πr^2 = 3.14*(2.34/2)*(2.34/2) = 4.67sqm

The volume of Tank required = 5000liters = 5cum.

Height of Tank = 5/4.67 = 1.07m,  Add free board 0.3m, therefore 1.07+0.3 = 1.37m

Hence, The Total capacity of the Circular tank with Freeboard is 6404.7 liters = 1690.8 gallons.

    Dia of Tank (Inner Dimension) = 2.44m

    Height of Tank(Inner Dimension) = 1.07(h) + 0.3(F.B) = 1.37m.


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