What is the value of the ratio of a circle's circumference to its diameter?

• A. e
• B. pi
• C. phi
• D. gamma

Answer: (B)

The ratio of a circle’s circumference to its diameter is a constant called pi. This ratio stays the same for every circle because both circumference and diameter scale together as the circle grows or shrinks. Using the standard formulas, circumference C = 2πr and diameter d = 2r, so C/d = (2πr)/(2r) = π. Numerically, π is about 3.14159.

Other constants like e, the golden ratio φ, or gamma are different quantities used in other areas of math and science, and they don’t describe this circle ratio.