Square feet is a term that applies to the measurement of the area of a shape. The perimeter of a shape is the length of the outer border of a shape. You can calculate the area of a figure when given the perimeter if that figure is a square or a circle. You may be required to perform this type of calculation in a high school or college math class. You also might find these formulas useful if you are measuring a space for a project such as landscaping or remodeling.
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 Calculator
Square

Divide the perimeter of a square by 4. For example, if the perimeter of a square is 20, you would divide 20 by 4 to get 5, which is the length of each side.

Multiply your answer from step one by itself to find the area of the square. This is also known as "squaring" a number. In this example, you would multiply 5 by 5 to get an area of 25.

State your answer in units squared. In this example your answer would be 5 square feet or 5 feet^2.
Circles

Divide the perimeter (which is also known as the "circumference") by pi, which is 3.14. For example, if you have a circle with a perimeter of 6 feet, you would divide 6 by 3.14 to get 1.91. This answer is the diameter of the circle.

Divide the diameter of the circle by 2 to get the radius. In this example, you would divide 1.91 by 2 to get 0.955.

Square the radius. In this example, you would multiply 0.9555 times 0.9555 to get 0.91.

Multiply your answer from step 3 by 3.14 to find the area. In this example you would multiply 0.91 times 3.14 to get an area of 2.8574 square feet.
References
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