How we can measure the quantity of water in a horizontal cylindrical tank?

1. V = A x h, area of cylinder is pi times radius squared.

2. Installed Magnetic level transmitter4-20mA and connect the Feedback signal to PLC & Scada.

1) By  level measurement for a homogeneous tank

2) By weight measurement, weight = mass x gravity = density x volume x gravity

3) By pressure measurement for a homogeneous tank, pressure = force / area = mass x gravity / area

With appropriate and correct calibration, you can get the quantity measured

http://www.aqua-calc.com/calculate/volume-in-a-horizontal-cylinder all you need

you can roughly get by measuring the heigth of your cylindrical tank and compute the area...V = A x h, area of cylinder is pi times radius squared.

we can use alot of methods like magnetic level indicator which give result to plc or whatever or use pressure transmitters solution p=rho g h like seal pot solution any way if that cannot be applcable can use2 wires naked that connect or not by liquid level if it is conducted liquid 

level transmitter is the best choice but if u cant afford it then one even can use flooting switches (digital, for extimation only).