The formula is
r², and to find r, or radius, you divide the diameter by 2. Your radius is 12.5, so you just plug that into your formula, and you have pi(12.5)², and simplify that to pi(156.25) and then your answer is
490.873852123. You probably want to round to the nearest hundredth, I don't know if you have specific directions, but if so your final answer would be 490.87.