Acres and miles are entirely different measurements; it is not possible to convert acres to miles. Acres are used to measure area, and miles are used to measure distance. Miles can be converted to yards, feet or inches because all three are measures of distance.
In order to relate two different increments of measurements, they both must be units of the same type of measurement. Using the distance measurement of one mile to define the borders of a space creates an area that is one square mile. Square miles and acres are both measurements of area. One square mile is equivalent to 640 acres.