Question 984571
A formula derived using two different drawn diagrams, and some algebraic steps, all done on paper;  difficult to show here on the site.


h, tank height
r, base radius
y, height of the water in partly filled tank
x, the radius at top of the liquid, used in the derivation of formula process


The work and result was done all in symbols.
Result that way  {{{(1/3)h*pi*r^2-(1/3)(h-y)pi*(r(h-y)/h)^2}}}-----still needs to be simplified a bit.