volume of liquid in bottom of a vertical cylindrical tank with parabolic bottom

Hamid... If you have a background in Calculus, I can provide you with the procedure.

The shape, if you don't already know is called a Circular Paraboloid. But, to solve it you have to provide some dimensional detail in order to "find" the constants of the parabola equation which takes the form, AX^2+BX+C.

An aside for those interested:

a) anyone know the difference between a parabola and a catenary?

b) can you provide examples where they are used in everyday objects?

Regards, Phil
Either Excel or integral calculus should work fine. Divide the solid up in discrete elements like concentric cylinders or stacked plates and add up the volumes.