Once you know the formula "Area = Pi(R squared)," calculating the area of a circle is simple. You'll need a ruler or measuring tape if you don't know the size of the circle you are working with. Grab a calculator or paper and pencil and sharpen those math skills.
Decide what value of pi to use to find the area of the circle. Pi can extend to an infinite number of places after the decimal without repeating. Your calculation will be more precise if you carry pi to several numbers beyond the decimal, but this may not be necessary for your purposes. In many cases, using 3.14 is adequate.
Find the radius of the circle, which is the distance from the middle point of the circle to the edge. If you know the diameter, divide it in half to get the radius. If you know the circumference -- the distance around the circle -- find the radius by dividing the circumference by pi and then by two.
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Find the square of the radius by multiplying it by itself. For example, if the radius is 10 feet, multiply 10 times 10 for a result of 100 square feet. Once you have squared the radius, multiply it by pi to find the area of the circle. In the example, multiply 100 times 3.14 for the resulting area of 314 square feet. This answer is the area of the circle.