The distance around a circle is called its circumference. The distance across a circle through its center is called its diameter. We use the Greek letter  (pronounced Pi) to represent the ratio of the circumference of a circle to the diameter.  we learned that the formula for circumference of a circle is: . For simplicity, we use  = 3.14. We know  that the diameter of a circle is twice as long as the radius. This relationship is expressed in the following formula: .