The perimeter of a circle is called the circumference.

The circumference is in a fixed ratio to the diameter. The ratio is represented by the Greek letter π (pi).

C = πD

π has an approximate value of 3.142 or 3.14 or 3, depending on the degree of accuracy you need to work to (but 3 is a poor approximation in most practical contexts).

This circle has a diameter of 10 cm. The circumference is 3.14 × 10 = 31.4 cm.