Diameter of the circle divided by 2 is the radius. Then use this simple formula.
Pi times radius squared pi is 3.14, Let's say that we are using 1 square mile and your circle has a diameter of 5280 feet, or one (1) mile across. 5280 divided by 2 = 2640 x 2640 = 6,969,600 square feet
6,969,600 x 3.14= 21,884,544 square feet in your circle
one square mile is 5280 x 5280 = 27,878,400 square ft (length times width)
now the simple math. 21,884,544 divided by 27,878,400 = .785 square miles in your circle
Pi times radius squared pi is 3.14, Let's say that we are using 1 square mile and your circle has a diameter of 5280 feet, or one (1) mile across. 5280 divided by 2 = 2640 x 2640 = 6,969,600 square feet
6,969,600 x 3.14= 21,884,544 square feet in your circle
one square mile is 5280 x 5280 = 27,878,400 square ft (length times width)
now the simple math. 21,884,544 divided by 27,878,400 = .785 square miles in your circle