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Circle calculator
Area, circumference, radius and diameter of a circle from any one of them.
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About Circle calculator
Give it any one of radius, diameter, circumference or area and it works out the other three. It uses C = 2 x pi x r and A = pi x r squared. A common slip is feeding in the diameter where the radius belongs, which quadruples the area.
How to use Circle calculator
- Pick which value you know: radius, diameter, circumference or area.
- Type that value into its field.
- Read the other three values, which are filled in for you.
Why use this tool?
- Private: everything happens in your browser. Nothing you enter is uploaded.
- Free, with no sign-up, no watermark and no limit on how often you use it.
- Works on phones, tablets and computers: Android, iPhone, Windows, Mac and Linux. Nothing to install.
- Free to embed on your own website, with a ready-made code to copy.
- Available in 10 languages.
Also known as
Frequently asked questions
How do I find the radius from the area?
Divide the area by pi, then take the square root. For an area of 78.54, 78.54 divided by 3.1416 is 25, and its square root gives a radius of 5.
What is the difference between radius and diameter?
The diameter is the full width of the circle through its centre, and the radius is half of that. If you measured across the circle, enter it as the diameter to avoid a result that is twice too big.
Which value of pi does a circle calculator use?
It uses the full precision pi built into the browser, about 3.14159265, not just 3.14. Your result may therefore differ slightly from a hand calculation done with 3.14 or 22/7.
Is Circle calculator free?
Yes. Circle calculator on Toolipedia is free to use, with no sign-up and no watermark.
Is it safe to use?
Yes. Everything is worked out in your browser, and nothing you enter is sent to our server.
Does it work on my phone?
Yes. It works in any modern browser on Android and iPhone, as well as on tablets and computers. There's nothing to install.
Do I need an account?
No. Just open the page and use the tool. There's no sign-up and no email needed.
Can I put Circle calculator on my own website?
Yes. Use the Embed on your site button under the tool to copy a short piece of code and paste it into your page. It is free, needs no account, and the tool keeps working inside your page.