Circle Calculator
Find radius, diameter, circumference and area.
How to use
- 1 Choose which value you already know: radius, diameter, circumference or area.
- 2 Enter that value.
- 3 Read the other three properties in the results panel.
- 4 Copy the results if you need them elsewhere.
About Circle Calculator
The Circle Calculator solves every property of a circle from a single measurement you already know.
Pick whether you are starting from the radius, the diameter, the circumference or the area, type the value, and the tool immediately works out the remaining three.
It is the fastest way to move between any pair of circle measurements without juggling the formulas yourself.
Under the hood it uses the standard relationships: the diameter is twice the radius, the circumference is two pi times the radius, and the area is pi times the radius squared.
When you supply a circumference it divides by two pi to recover the radius, and when you supply an area it takes the square root of the area over pi.
Because the radius is computed first and everything else derives from it, the four values are always perfectly consistent with one another.
Results update live as you type and are shown together in a clear results panel you can copy in one tap.
It is useful for geometry homework, engineering and DIY measurements, drawing and CAD work, or any time you need to convert quickly between a circle’s size descriptors.
Everything is calculated locally in your browser with no uploads, no tracking and no sign-up.
FAQ
Which formulas does it use?
Diameter = 2r, circumference = 2πr and area = πr². From circumference it recovers r = C / (2π); from area it uses r = √(A / π).
Why must the value be greater than zero?
A circle needs a positive size. Zero or negative inputs have no valid radius, so the tool asks for a value above zero.