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How this calculator works
A circle is fixed by a single number. Knowing the radius, the diameter, the circumference or the area lets you compute the other three, all through the constant π, the ratio of a circle's circumference to its diameter. The circumference and diameter come out in the unit you entered and the area in that unit squared.
Formula
- Diameter = 2 × radius
- Circumference = 2 × π × radius
- Area = π × radius²
- From area: radius = √(area / π)
Worked example
Radius 5
- I know the
- Radius
- Value
- 5
ResultArea 78.5398
Circle with radius 5 and circumference 31.4159.
Frequently asked questions
How do I find the area of a circle from its diameter?
Halve the diameter to get the radius, square it and multiply by π. A diameter of 10 gives π × 5² ≈ 78.54.
What is pi?
The ratio of a circle's circumference to its diameter, approximately 3.14159. It is the same for every circle.
How do I find the radius from the circumference?
Divide the circumference by 2π.
Sources
More in Math
Results are estimates based on the values you enter and the published method shown above. They do not replace professional medical, financial or legal advice. Method reviewed October 7, 2026. About this site