**Circumference (C):**The closed line of a circle — that is, the distance around the circle — is called its circumference.**Radius (r)**: The radius of a circle is the measurement from the center of the circle to any point on the circumference of the circle.**Diameter (d)**: The diameter of the circle is measured as a line passing through the center of the circle, from a point on one side of the circle all the way to a point on the other side of the circle.

The diameter of a circle is always twice as long as the radius of a circle: *a=2r*.

## Navigating the circumference

To measure the circumference of a circle, use the number pi (π). Although π is a lengthy number, it’s generally rounded to 3.14 or 22/7. If you round π so you can solve a problem, the equal sign isn’t used because the answer isn’t equal to the actual length. The approximation symbol (≈) is used.The formula for circumference is circumference = π × diameter, or C = πd. Because the radius of a circle is half its diameter, you can also use the radius to determine the circumference of a circle. Here’s the formula: C = 2πr.

Suppose you know that the pie you just baked has a diameter of 9 inches. You can determine its circumference by using the circumference formula:*C* = π*d*
*C* ≈ 3.14 × 9
*C* ≈ 28.26 inches

## Mapping out the area

Determining the area of a circle also requires the use of π.*Area*= π

*x*the square of the circle’s radius, or

*A*= π

*r*

^{2}.

To determine the area of a 9-inch-diameter pie, multiply π by the square of 4.5. Why 4.5 and not 9? Remember, the radius is always half the diameter, and the diameter is 9 inches.

*A* = *xr*^{2
}*A* = 3.14 × 4.5^{2
}*A* = 3.14 × (4.5 × 4.5)
*A* = 3.14 × 20.25
*A* = 63.585 inches