How to calculate the required bandwidth from a spectrum diagram? [FM]

For example from this diagram.

Answer

There are many definitions of bandwidth; see for example section 2.2 in this book (PDF linked at the end of the page). In your case, it is easy to use the "absolute bandwidth", which is defined as $B=f_{max}-f_{min}$, where $f_{max}$ is the largest frequency in the spectrum, and $f_{min}$ is the smallest.

According to this definition, you'd have $B=(100.028-99.972)\mathrm{\,MHz}=56\mathrm{\,kHz}$.