To find the perimeter of a circle, also known as the circumference, plug the dimensions of the circle into the formula ((pi)(d). This formula stands for pi (approximately 22/7) times the diameter of the circle (d).
Pi is an irrational number, which means it never ends. In math, 3.1415 is used as a general approximation of pi. The diameter is simply the length of the line that cuts through the center of the circle. This formula can also be written as 2 pi r. The radius (r) is simply half of the diameter, so multiplying it by two gives the diameter, making this equation equivalent to the first.