What is the Circle Calculator?
The circle calculator finds the area, circumference and diameter from the radius. It is useful for school geometry, design and construction work.
How to use the Circle tool
- Enter Radius.
- The result updates instantly as you type, or press Calculate.
- Use Try an example to see how it works, and Reset to start again.
Formula
Area = π × r². Circumference = 2 × π × r. Diameter = 2 × r. Here π is about 3.14159.
Worked example
With radius 5, the area comes to 78.5398. Circumference: 31.4159; Diameter: 10.
Frequently asked questions
How do I find area from the diameter?
Halve the diameter to get the radius, then use area = π × r².
What is pi?
Pi is the ratio of a circle circumference to its diameter, about 3.14159.
Circle measurements by radius
Diameter = 2r, circumference = 2πr, area = πr².
| Radius | Diameter | Circumference | Area |
|---|---|---|---|
| 1 | 2 | 6.283 | 3.142 |
| 2 | 4 | 12.566 | 12.566 |
| 3 | 6 | 18.85 | 28.274 |
| 4 | 8 | 25.133 | 50.265 |
| 5 | 10 | 31.416 | 78.54 |
| 6 | 12 | 37.699 | 113.097 |
| 7 | 14 | 43.982 | 153.938 |
| 8 | 16 | 50.265 | 201.062 |
| 10 | 20 | 62.832 | 314.159 |
| 12 | 24 | 75.398 | 452.389 |
| 15 | 30 | 94.248 | 706.858 |
| 20 | 40 | 125.664 | 1,256.637 |
FAQ
How do I find area from circumference? Divide the circumference by 2π to get r, then compute πr².
What value of π is used? 3.14159265 (full double precision).
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Checked by Calcovate · Formula tested against worked examples · Updated 7 Nov 2026 · How we check our calculators.