An acre is a unit of area, like square feet or square yards, most commonly used to measure large tracts of land. The usage of the acre unit dates back as early as the 8th century in England. In order to calculate the number of acres in a circle, you need to know the area of a circle can be found using the formula radius times radius times pi. Pi is the ratio between the diameter of a circle and the circumference.
Measure the distance from the center of the circle to the edge of the circle. This is the radius.
Multiply the radius by itself. For example, if the radius is 300 feet, multiply 300 feet by 300 feet to get 90,000 feet squared.
Multiply the previous step's result by pi, which is approximately 3.1415, to find the area of the circle. In this example, multiply 90,000 by 3.1415 to get 282,735 square feet.
Convert the area of the circle to acres. If you measured the area in square feet, divide by 43,560. If you measured the area in square yards, divide by 4,840. In this example, divide 282,735 by 43,560 to get 6.49 acres.