Understand pi, its approximate value and its use in circumference, area and volume calculations.
Pi is the circumference of a circle divided by its diameter. Its decimal expansion begins 3.141592653589793. It is irrational: no fraction of integers gives its exact value and its decimal expansion never terminates or repeats.
For radius r and diameter d = 2r, circumference is 2πr or πd; area is πr². A circle with radius 3 m has circumference 6π ≈ 18.849556 m and area 9π ≈ 28.274334 m². Circumference and area have different units.
Writing 9π m² preserves an exact symbolic result. Using 3.14 gives 28.26 m², while a calculator retaining more digits gives about 28.274334 m². The useful precision depends on how accurately the radius was measured.
Using diameter where radius is needed multiplies circle area by four. Doubling a radius doubles circumference but quadruples area. For a cylinder, multiply the base area πr² by height to obtain volume.
Editorial responsibility: Nicolas Belotti · Methods and testing · Report an error
English edition: 16 September 2026.