# Circumference of a Circle Lesson

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## Circumference of a Circle

*(pronounced Pi)*to represent this value. The number goes on forever. However, using computers, has been calculated to over 1 trillion digits past the decimal point.

The distance around a circle is called the **circumference**. The distance across a circle through the center is called the **diameter**. is the ratio of the circumference of a circle to the diameter. Thus, for any circle, if you divide the circumference by the diameter, you get a value close to . This relationship is expressed in the following formula:

where is circumference and is diameter. You can test this formula at home with a round dinner plate. If you measure the circumference and the diameter of the plate and then divide by , your quotient should come close to . Another way to write this formula is: where **·** means multiply. This second formula is commonly used in problems where the diameter is given and the circumference is not known (see the examples below).

The **radius** of a circle is the distance from the center of a circle to any point on the circle. If you place two radii end-to-end in a circle, you would have the same length as one diameter. Thus, the diameter of a circle is twice as long as the radius. This relationship is expressed in the following formula: , where is the diameter and is the radius.