Circle area and circumference calculator
Find circle area and circumference from radius or diameter. Choose a unit and see the formula with your values.
How to use this page
- Load an example or enter your own data.
- Choose the calculation type and units where available.
- Read the result, additional measures and substituted formula.
What this page does not do: Length must be positive and at most one billion. Changing the unit relabels the input; it does not convert its numeric value. This model describes a circle on a plane.
Further reading
Formula
A = πr²; C = 2πr; d = 2r Area measures the region inside a circle; circumference measures its boundary. Diameter is twice the radius. When you enter a diameter, the calculator divides it by two first.
For radius 3 cm, area is 9π cm², about 28.274334 cm²; circumference is 6π cm, about 18.849556 cm. Doubling the radius doubles circumference and quadruples area.
Frequently asked questions
How can I check the example?
Example r = 3 cm: area 9π cm², circumference 6π cm.
What should I check before interpreting the result?
Length must be positive and at most one billion. Changing the unit relabels the input; it does not convert its numeric value. This model describes a circle on a plane.
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