This circle calculator finds the radius, diameter, circumference and area of a circle from any one of them. Choose what you know and enter its value.
Circle formulas
- Diameter: d = 2r
- Circumference: C = 2πr = πd
- Area: A = πr²
- Radius from area: r = √(A ÷ π)
Worked example
A circle with a radius of 5 has a diameter of 10, a circumference of about 31.4159 and an area of about 78.5398 square units.
Frequently asked questions
What value of π does the calculator use?
It uses the full precision available in your browser (3.141592653589793), and rounds results to four decimal places.
How do I find the area from the diameter?
Halve the diameter to get the radius, then use A = πr². The calculator does this for you if you choose “Diameter”.