How to calculate water tank capacity in liters?

Let us suppose your tank dimensions are

L = 8ft
B = 6 ft
D/H = 5 ft
Then your volume of 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 x 28.31 = 6794.4 liter.
That means almost 6800 liter of water can be stored in the tank.

If you have dimension in meter say
L = 2.5m
B = 2.5m
D/H = 2m
Then volume shall be 2.5 x 2.5 x 2.0 = 12.5 cubic meter.
Now, 1 cubic meter = 1000 liter,
Hence, total volume of water in tank say water tank capacity shall be
12.5 x 1000 = 12500 liters.

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