Because pi represents the ratio of a circle's circumference to its diameter
pi has two interesting aspects.
One, it is an infinite equation, it has no end as far a we have been able to calculate, most people stop a 3.14 or 3.1416 but for accuracy most will use 3.141592. "Pi" is a much larger calculation however.
Two, it applies to every circle and every form of measurement. It works with standard and metric measurements. Just multiply the diameter by "pi" and you get the circumference.
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Hope it helps
Ranjan Kumar