Hello.I need to add chlorine to an OVERHEAD water tank.for that I need to know the proper quantity of chlorine to be added.and for that I need to know the tank capacity.unfortunately I do not know the dimensions of the tank,neither I have drawing of it.. BUT... a vertical inline pump sucks water from an underground tank and fills this OVERHEAD TANK.. even the rated flow of this pump is unknown.. how to CALCULATE the tank size?

The water level in the OHT can be found out using a home mode wooden dip stick(graduated with level marks)

Not possible to determine the volume without knowing the dimensions. How can you not know the dimensions if you can determine the water level with a dip stick. If you can measure the water level with a dip stick you can determine at least one dimension with a dip stick.

I suppose you could measure the water level and then drain and measure the amount of water drained to determine the total capacity of the tank.