The conversion of a given diameter to square feet uses a simple formula for determining the area of a circle. This can be useful for deciding how much paint to buy to cover a circular ceiling or perhaps how much grass seed to purchase for a circular-shaped patch of lawn. The formula for the area of a circle is the radius squared multiplied by the constant pi, or 3.14159. Many calculators include the constant pi on one key, eliminating the need to remember it.
Measure the diameter of the circular area in feet using the tape measure. The diameter is a straight line that passes through the center of a circle. For example, assume the diameter of the circular area to be 12 feet.
Divide the length of the diameter by 2 to find the radius (12 / 2 = 6). The radius is 6 feet.
Multiply the radius by itself to square the number (6 x 6 = 36). Multiply the result by pi (use the button on the calculator) or 3.14159 (36 x 3.14159 = 113.1). The result is the area of the circle in square feet--113.1 square feet.
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An alternate formula is the diameter squared divided by 4 with the result multiplied by pi, or 3.14159.