The Oxford Dictionary defines the circle is, “a round plane figure whose boundary (the circumference) is equidistant from a fixed point.” Wikipedia calls it “a simple closed shape.” In his treatise, “The Elements,” Euclid defined it this way: “A circle is a plane figure bounded by one line, and such that all right lines drawn from a certain point within it to the bounding line, are equal. The bounding line is called its circumference and the point, its centre.” Using circles and learning how to calculate values for circles has played an essential part in the evolution of our civilization, from the development of the wheel and the abacus with its round beads to the evolution of modern machinery and mathematics. **Here are some basic explanations for calculating the values for the area, radius, diameter, and circumference of circles.**

## Solving for Area

The area of a circle is the interior that is enclosed by the circle. To determine the area, if you know the radius, use the formula *A = **π* *r*^{2}, where *A* is the area and *r* is the radius. The Greek letter *π* represents 3.142159, which is a constant defined by the ratio of the circumference of any circle to its diameter. If you don’t know the radius but know the diameter, divide the diameter value in half to obtain the radius. If you know the circumference of the circle, use the formula *A = C*^{2}* / 4**π*, where *C* is the circumference.

## Solving for Radius

The radius of a circle is a straight line passing from the center of a circle to its circumference. You can find the radius by dividing the diameter in half. If the diameter is unknown, you can find the radius using any other known measurement of the circle, such as area, diameter, or circumference. For example, if you know the area of the circle, solve for the radius *r*.

## Solving for Diameter

The diameter is a straight line starting at one point on the circle, passing through the center of the circle, and ending at another point on the circle. If you know the radius of the circle, simply multiply the value by two. If you know the area of the circle, divide the result by *π*, obtain its square root to find the radius, and then multiply the value of the radius by 2 to find the diameter. If you know the circumference of the circle, divide the value by *π* (*π* is approximately 3.14).

## Solving for Circumference

The circumference of a circle is the distance around the outside of the circle. If you know the diameter of the circle, you can find the circumference using the formula, *C = π d,* where *d* is the diameter (π is approximately 3.14). If you know the radius, double the value to obtain the diameter.

From stone circles and crop circles to surveillance video technology and communication satellites orbiting the earth, circles have played a vital role in all aspects of our history and our civilizations.

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