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 Description
Circumference:  The distance around a circle.
 The circumference of a circle can be found using the formulas C = 2*pi*r or C = pi*d (* means to multiply, r = radius, and d = diameter). The circumference (indicated in red) of a circle is similar to the perimeter of a polygon in that both measure the distance around a figure. As the number of sides of an inscribed regular polygon increases (see figures below), the perimeter of the regular polygon gets closer to the circumference of the circle. In other words, the perimeter of the regular octagon more closely approximates the circumference of the circle than does the perimeter of the regular pentagon or the equilateral triangle. The ratio of the circumference of any circle to its diameter is always a number that is close to 3.1416. This number is called pi and is written . Pi is an irrational number, which means it cannot be written as an exact decimal. We usually estimate pi with 3.14, 3.1416, or 22/7.