Let's look at an example. If d = 4, then C = πd = 4π.
C = 12.56637061
or 12.56637061... = 4π
Now, if you solve for π, π = 3.14159... - which is π
An irrational number is one that never terminates and never repeats.
So, when you divide any circumference by its diameter, you will always get π.