Pi is the ratio of a circle’s circumference to its diameter. In formula form, this is:

π=circumference of a circlediameter of that circle\pi =\frac{\text{circumference of a circle}}{\text{diameter of that circle}}π=diameter of that circlecircumference of a circle​

So if you take any circle, measure all the way around it (the circumference), then divide by the distance straight across through the center (the diameter), you always get the same number: π≈3.14159\pi \approx 3.14159π≈3.14159.