Though it may seem strange to say, circles are measured in square units. The area of a circle requires squaring its radius, which is a straight line from its origin, or center coordinates, to its rim, or circumference. Multiplying a unit of measurement to itself results in that unit being square; when multiplying a foot by a foot, the result is a square foot. Given that a circle's area is in square units, it can be converted to square feet from other square measurements.

Measure the length of the diameter. The diameter is a straight line that travels from one point on the circle's circumference, through its origin, and to a second point on the circumference. For this example, the diameter is measured at 8 yards.

Halve the length of the diameter to calculate the length of its radius. For this example, half of 8 yards is 4 yards.

Square the length of the radius. For this example, 4 yards squared becomes 16 square yards.

Multiply the amount of the radius squared by pi to calculate the area in square yards. Pi is a mathematical constant that begins as the number 3.14 and never terminates. For this example, 16 square yards multiplied by pi equals 50.265 square yards.

Multiply the amount in square yards by 9 to convert the area to square footage. For this example, the amount of 50.265 in square yards multiplied by 9 converts to 452.385. Click on the Metric Conversion Chart link (see resources) to see how to convert other common square amounts to square meters. For example, 1 square foot equals 144 square inches and 1 square meter equals 10.764 square feet.