We know the circumference of a circle is 2 * pi * Radius (c = 2 * pi * r). The diameter of a circle is 2 * Radius (d = 2 * r). Therefore the circumference of a circle is pi * Diameter (c = pi * d). The ratio between circumference and diameter is pi, which is a constant and therefore doesn’t change even when radius size changes.

How do we know Pi? We have literally known about it for so long that no one has an historical account of who first conceived of it. The oldest example we have is from Babylon, and even then we don’t think they discovered it just that they were aware of it. www.britannica.com/science/pi-mathematics