The circumference of a circle is the linear distance around the edge of the circle, also known as the perimeter of the circle. It is measured in units of length such as feet, inches, centimeters, meters, miles, or kilometers. The circumference of a circle can be calculated using the formula C = 2πr or C = πd, where r is the radius of the circle and d is the diameter of the circle. The constant π (pi) is a special mathematical constant with a value approximated to 3.14159. It is impossible to find the exact value of π because it is an irrational number. Therefore, the circumference of a circle can only be approximated. To calculate the circumference of a circle, you can use a circumference calculator or solve the formula manually.