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The Number Pi (π)

π ≈ 3.14159 — the ratio of circumference to diameter. Formulas and practice trainer.

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Pi — Key Facts

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Frequently Asked Questions

What is the number pi?

Pi (π) is the ratio of a circle's circumference to its diameter. It is an irrational number: π ≈ 3.14159265358979...

What is pi used for?

Circumference C = 2πr, circle area A = πr², sphere volume V = (4/3)πr³.

How many digits does pi have?

Pi is irrational — its decimal expansion never ends and never repeats. In practice, 3.14 or 3.14159 is enough for most problems.