EVERYDAY CALCULATORS
Circle Calculator — Area, Circumference & Diameter
Calculate a circle's area, circumference, and diameter from its radius using pi. Instant, in your browser, private, no signup.
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Enter a circle's radius and get its area, circumference, and diameter at once, using a precise value of pi rather than a rounded 3.14.
The circle formulas
| Quantity | Formula | Radius 5 gives |
|---|---|---|
| Diameter | d = 2r | 10 |
| Circumference | C = 2πr | ≈ 31.42 |
| Area | A = πr² | ≈ 78.54 |
Area depends on the square of the radius, so doubling the radius quadruples the area, while circumference only doubles.
Frequently asked questions
- What if I only know the diameter?
- Halve it to get the radius (r = d / 2), then apply the area or circumference formula. The diameter is always exactly twice the radius.
- What value of pi does it use?
- The browser's built-in high-precision pi (about 3.14159265358979), not a rounded 3.14, so any rounding you see is only in how the result is displayed.