When you plan to make a water tank, the finalize the dimension is very important to get the exact quantity of water stored. You must keep in mind to keep minimum dimension excluding water proof plaster to get the right amount of water storage quantity. In reality, haunches to be deducted as well, but, lets have some margin to take care of it.

Let us suppose your tank in to in dimensions (excluding plaster) are

L = 8 ft, B = 6 ft, D/H = 5 ft

Then, volume of the tank will be 8 x 6 x 5 = 240 ft³

Now 1ft³ = 28.31 liter. hence, total volume of tank in liter shall be

240 ft³ x 28.31 (conversion factor) = 6794.4 liter.

That means almost 6800 liter of water can be stored in the tank.

Now, let us calculate the storage capacity if the dimensions are in meter unit.

L = 2.5 m, B = 2.5 m, D/H = 2.0 m.

Then, volume of the tank shall be 2.5 x 2.5 x 2.0 = 12.5 m³.

Now, 1 m³ = 1000 liter. hence, total volume of water in tank say water tank capacity shall be

12.5 m³ x 1000 (Conversion factor) = 12,500 liters.

That means 12,500 liter of water can be stored in the tank.

Here is a quick video to show you How to calculate water tank capacity in liters?