The Circumference Formula
The circumference of a circle (the distance around its edge) is calculated as C = 2πr, where r is the radius, or equivalently C = πd, where d is the diameter. Since d = 2r, both formulas are identical.
For a circle with radius 5 cm: C = 2π × 5 = 10π ≈ 31.42 cm. For a circle with diameter 12 inches: C = π × 12 = 12π ≈ 37.70 inches.
π (pi) is the ratio of a circle's circumference to its diameter — a constant approximately equal to 3.14159265. It is irrational (its decimal never terminates or repeats) and transcendental (cannot be expressed as a root of any polynomial). The first 10 digits are 3.1415926535.