Divide the diameter by 2 to get the radius and square the result. Then multiply that result by your value for .
Hint: Change feet and inches to inches before doing the calculation so that you don't have to mess with trying to square a number with mixed feet and inches. Your answer will then come out in square inches. Divide by 144 after that if you want it in square feet.
My calculator said it, I believe it, that settles it