Circle Area & Circumference Calculator

Given either the radius or diameter, the other is derived first (diameter = 2 × radius), then the standard formulas apply: area = π × radius², and circumference = 2 × π × radius (equivalently, π × diameter).

The diagram below is a schematic, not drawn to literal scale (an actual radius can be far too small or large to draw at real size) — it's there to make the radius-vs-diameter relationship concrete, since mixing the two up is a common source of area/circumference errors.