Hi
Through the Pythagorean theorem the radius of the circular section is $\displaystyle R = \sqrt{125^2-30^2}$
The section is $\displaystyle \pi R^2 = \pi (125^2-30^2)$
Thank you. Love that Ptyhagorus...what a guy! Thats a great help as I needed to find section diameter at a few different levels as I am building something inside a sphere