Circle calculator
A circle is fully defined by a single measurement. Enter a radius, diameter, circumference, or area and this calculator returns all four properties.
Radius
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Diameter
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Circumference
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Area
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Formulas
| Property | Formula |
|---|---|
| Diameter | d = 2r |
| Circumference | C = 2πr = πd |
| Area | A = πr² |
Example
Radius = 5:
- Diameter = 10
- Circumference = 2π × 5 ≈ 31.416
- Area = π × 25 ≈ 78.540
References
- Circle — area (πr²) and circumference (2πr)Wikipedia · en.wikipedia.org
- Circle — formulas and derivationsWolfram MathWorld · mathworld.wolfram.com
Frequently asked
What is the formula for the area of a circle?
Area = π × r². For a radius of 5, Area = π × 25 ≈ 78.54.
What is the formula for circumference?
Circumference = 2πr = πd. It is the perimeter of the circle.
How do I find the radius from the area?
Rearrange Area = πr² to get r = √(Area / π).
How do I find the radius from the circumference?
Rearrange C = 2πr to get r = C / (2π).
What units does this calculator use?
Whatever unit you enter the value in. If you enter 5 cm, all outputs are in cm (or cm²).
How do I share my calculation?
Click "Share with my numbers" to copy a URL that restores your inputs.
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