What does this tool do?
The Circle Calculator instantly computes the remaining three values when you enter any one of radius, diameter, area, or circumference. It is a fast, reliable geometry helper for students, teachers, engineers, and designers.
How to use it
- Enter your values into the fields.
- The result is calculated instantly.
- Read or copy the result.
Which value to start from
Pick the known value (radius, diameter, area, or circumference) from the dropdown and type the number. The tool first derives the radius: from diameter r = diameter / 2, from area r = √(area / π), and from circumference r = circumference / (2π). It then computes all four results: radius, diameter (2r), circumference (2πr), and area (πr²). For example, entering a radius of 5 gives a circumference of about 31.4159 and an area of about 78.5398.
Where it is useful
It comes up often in real life: estimating fabric area for a round tablecloth, fencing the perimeter of a circular garden bed, comparing pizza sizes, or finding the cross-sectional area of a pipe. Results show up to four decimal places with trailing zeros trimmed automatically, so you get clean values. The copy button next to each result lets you grab a value in one click and paste it into homework, a drawing, or a spreadsheet.
Frequently asked questions
Can I find the area if I only know the circumference?
Yes. Choose circumference as the known value and enter it; the tool first works back to the radius, then displays the area, diameter, and radius.
Which value of pi does it use?
The calculations use JavaScript's Math.PI constant, which is 3.141592653589793, so you get high-precision results.
Why are results limited to four decimals?
For readability, values are rounded to four decimal places and trailing zeros are removed. Results that come out as whole numbers are shown without decimals.
Is Circle Calculator free?
Yes. Circle Calculator and all 300+ tools on MagmaNex are completely free with no signup or account required.
Is my data safe?
Yes. Everything runs locally in your browser. The data you enter is never sent to a server, stored, or shared.
Does it work on mobile?
Yes. Circle Calculator works on phones, tablets and desktops — no app download needed.
🔒 Privacy: everything runs in your browser, your data is never sent to a server.