The original question was: If the diameter of a circle is a fixed number, say 10cm, why does Pi go on for billions of numbers even though it extrapolates from the diameter’s value? Why is it not a definite number? There is a very simple relationship between diameter and circumference, given by circumference =diameter. This occurs because, well, that is a property that circles have.is however a “definite” number, in the sense that it is a single fixed number that is well defined. with an infinite number of zeros.