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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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About this calculator

Formulas

PropertyFormula
Diameterd = 2r
CircumferenceC = 2πr = πd
AreaA = πr²

Example

Radius = 5:

  • Diameter = 10
  • Circumference = 2π × 5 ≈ 31.416
  • Area = π × 25 ≈ 78.540

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.