In this lesson, students learn that the circumference of a circle is the distance around the circle, and the formula for the circumference of a circle is 2 times pi times the radius of the circle. Pi is the ratio of the circumference to the radius, which is approximately 22/7 or 3.14. So a circle with a radius of 3 feet has a circumference of 2 times pi times 3, or 6 pi feet. And since pi = 3.14, 6 pi feet can also be written as 6 times 3.14, or 18.84 feet. Finally, note that the radius of a given circle is half the diameter of the circle. So to find the circumference of a circle with a given diameter, first find the radius of the circle by dividing the diameter by 2.