Rex says
The fast lane for the math you almost remember from school. Type the numbers, get the answer, move on with your day.
Try a scenario
Click to load — tweak from there.Inputs
Result
Area
314.1593
Radius
10.0000
Diameter
20.0000
Circumference
62.8319
Psst — share this and help Rex grow
One click, a permanent link with your numbers baked in.
How to use this
- 1Enter what do you know?.
- 2Enter value.
- 3Read your area on the right — it updates as you type.
- 4Hit Share to keep the scenario or send it to someone.
About this calculator
Enter whichever measurement you have and the rest follow. Useful for everything from pipe sizing to working out how much edging a round garden bed needs.
Worked example
Using the values the calculator loads with:
Inputs
- What do you know?: Radius
- Value: 10
Results
- Area: 314.1593
- Radius: 10
- Diameter: 20
- Circumference: 62.8319
What each field means
Inputs
- What do you know?
- Pick the option that matches your situation — the maths changes per option. Choices: Radius, Diameter, Circumference, Area.
- Value
- The value used in the calculation. Starts at 10 so you have a working example on load.
Results
- Area
- Returned as a decimal number and shown as the headline result. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Radius
- Returned as a decimal number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Diameter
- Returned as a decimal number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Circumference
- Returned as a decimal number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
FAQ
How does the circle calculator work?
Enter whichever measurement you have and the rest follow. Useful for everything from pipe sizing to working out how much edging a round garden bed needs. The underlying maths is: C = 2πr. A = πr².
What do I need to enter?
2 values: what do you know?, and value. Each field starts with a sensible default, so you can change one number at a time and watch the result move.
What does the area result mean?
It is returned as a decimal number and updates live as you edit the inputs, so you can compare two or three versions of a scenario in a few seconds.
Is this calculator free, and do I need an account?
Yes, it's free, and no account is required. Nothing you type is stored on our servers — the maths runs entirely in your browser.
How accurate is the result?
It applies the standard formula exactly, so the arithmetic is precise. Results are rounded for display; the underlying calculation keeps full precision.
Who is this tool for?
It's built for students, developers, and anyone who needs the answer right now without setting up a spreadsheet.
Accuracy and limitations
- Results are rounded for display; the underlying calculation keeps full precision.
- Very large or very small inputs may hit floating-point limits in the browser.
- Inputs outside the accepted range are clamped rather than rejected.
Related tools
Cite this calculator
Writing about this topic? Grab a citation — every link helps keep these tools free.
RevenueLab. (2026). Circle Calculator. Retrieved from https://www.revenuelab.fyi/toolbox/circle
<p>Source: <a href="https://www.revenuelab.fyi/toolbox/circle" target="_blank" rel="noopener">Circle Calculator — RevenueLab</a> (2026).</p>
Source: [Circle Calculator — RevenueLab](https://www.revenuelab.fyi/toolbox/circle) (2026).