The diameter quantifies the distance of a straight line from one point on a circle through the center to another point opposite it, and can be measured in feet. The area of a circle can be measured in square feet. Figuring the area can be useful if you need to calculate how much paint to buy or how much sod would to cover a lawn.

Consider the formula for the area of a circle: A = πr^2. The radius of a circle, the distance from the center to a point on the circumference, is one half of the diameter.

Divide the diameter, measured in feet, by two to find the radius of the circle. For example, if the circle's diameter equals 4.5 feet, divide 4.5 by 2 to get 2.25 feet for the radius.

Multiply the radius by the radius. Here, multiply 2.25 feet by 2.25 feet to get 5.0625 square feet.

Multiply the value for the radius squared by pi, commonly approximated at 3.14159, to find the area of the circle in square feet. Finishing the example, multiply 5.0625 by 3.14159 to find the area of the circle, about 15.904 square feet.