What type of number best describes the diameter of a circle

The diameter of a circle is a quantity which has a unit that is usually rounded to a certain terminating degree of accuracy (i. e. 1, 2, ..., n decimal places)

Thus the set of rational numbers best describes the diameter of a circle.

A rational number is a number which can be represented in the form a/b where a and b are integers. It is usually identified by a terminating or a recurring decimal number.