This circle calculator finds diameter, circumference, and area from a radius using standard plane-geometry formulas.
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Circle Calculator
Calculate radius, diameter, circumference and area of a circle.
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About this calculator
Formula notes
diameter = 2r, circumference = 2πr, area = πr². If you start from diameter, use r = d / 2.
Worked examples
- Example: r = 2 gives diameter 4, circumference ≈ 12.566, and area ≈ 12.566 — matching the page defaults.
- For a radius of 7 cm, diameter is 14 cm, circumference is about 43.982 cm, and area is about 153.938 cm2.
How to use it
- Enter the radius (use r = d / 2 if you only have the diameter).
- Confirm the unit, since all outputs inherit it.
- Read diameter, circumference, and area together to compare.
Frequently asked questions
Why do both circumference and area depend on π?
π is the constant ratio of a circle's circumference to its diameter, so both formulas derive from it.
Can this solve for radius from area?
Yes. From area A, the radius is r = √(A / π), and the page recomputes the other values.
Limits and interpretation
- The formulas assume a perfect Euclidean circle and do not account for measurement tolerance or an elliptical shape.
- Circumference uses linear units while area uses squared units; they are numerically similar for some radii but not interchangeable.