Circle Area & Circumference Calculator
Calculate a circle's radius, diameter, area, and circumference from either the radius or diameter.
- Radius
- 5
- Diameter
- 10
- Area
- 78.5398
- Circumference
- 31.4159
Radius and diameter
How it works
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.