So far, all of our solids of revolution have been obtained by rotating around the $x$ axis. If we have a solid of revolution around the $y$ axis, then the roles of $x$ and $y$ swap.
We slice horizontally, and our washers have thickness $\Delta y$ and inner and outer radii $g(y)$ and $f(y)$, where $f(y) \ge g(y) \ge 0$. The volume is then