Mathematics · Glossary
What is Pi (π)?
Quick definition
The ratio of a circle's circumference to its diameter. Approximately 3.14159, an irrational number.
Full explanation
Pi (π) is the ratio of a circle's circumference to its diameter, approximately 3.14159265358979. It is irrational (cannot be expressed as a fraction) and transcendental (not a root of any polynomial with integer coefficients). Pi has been calculated to over 100 trillion digits. It appears in formulas for circles, spheres, cylinders, trigonometry, Fourier transforms, normal distribution, and many areas of physics. Pi Day is celebrated on March 14 (3/14). Ancient cultures approximated pi as 22/7 (~3.143), 355/113 (~3.1415929 — accurate to 6 digits), and 3.1605 (Egyptians, ~1650 BC). For most practical purposes, 3.14159 is sufficient.
Related calculators
Calculators that use or explain Pi (π).
Related terms
More from Mathematics
Standard Deviation
A measure of how spread out numbers are from their mean. Low = consistent, high = volatile.
P-Value
The probability of observing results at least as extreme as those observed, assuming the null hypothesis is true.
Mean (Average)
The sum of values divided by the count. The most common type of "average." Sensitive to outliers.
Median
The middle value in a sorted list. Half the values are above, half below. Robust to outliers.
Pythagorean Theorem
In a right triangle, the square of the hypotenuse equals the sum of the squares of the two legs: a² + b² = c².
Logarithm
The exponent to which a base must be raised to produce a given number. log(100) = 2 means 10² = 100.
Last reviewed: June 15, 2026 • Category: Mathematics