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Circle Calculator

Enter a circle's radius, diameter, circumference or area to get all the others. Uses π to full precision. Free circle calculator.

Radius
5
Diameter
10
Circumference
31.415927
Area
78.539816

Every measurement of a circle from one

A circle is defined by one length, so knowing any one measurement gives you all the others. Diameter = 2 × radius, circumference = 2πr and area = πr². For a radius of 5: the diameter is 10, the circumference 31.416 and the area 78.540. Working backwards, a circumference of 100 means a radius of 15.915. Use any unit; the area comes out in that unit squared.

How to use it

  1. 1

    Choose what you know

    Radius, diameter, circumference or area.

  2. 2

    Enter the value

    In any unit.

  3. 3

    Read the rest

    Radius, diameter, circumference and area.

Frequently asked questions

How do I find the circumference?

C = 2 × π × r, or π × diameter.

How do I find the radius from the area?

r = √(area ÷ π).

What value of π is used?

JavaScript's Math.PI, 3.141592653589793.

Is anything stored?

No. The calculation runs in your browser.

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