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 do I 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 Theory — how are circle area and circumference calculated?
The area of a circle describes the surface enclosed within the circle's edge, while the circumference is the length of that edge. The diameter is always twice the radius, so if a diameter is entered, the calculator first divides it by two to obtain the radius. For a radius of 3 centimeters, the area equals 9π square centimeters and the circumference equals 6π centimeters. The calculator lets you choose a unit and shows a worked example with the values substituted into the formula.
Frequently asked questions
How can I check the example?
What should I check before interpreting the result?
See also
Ancient Egyptian units converter
Convert length, area, volume and mass both ways. A 0.525 m cubit, 4.8 L hekat and 91 g deben model; khar varia
Arithmetic Mean
Enter numbers separated by commas and calculate the arithmetic mean online. The calculator will give you the s
Basic calculator
Add, subtract, multiply and divide with parentheses and percentages.