Free circle calculator
Calculate circle area, circumference, and diameter — enter a radius and see all measurements instantly, updated live, as you type.
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C = 2πr, A = πr². Source: Euclidean geometry.
Results are estimates. Consult a professional.
How the circle calculator works
A circle is defined by a single measurement — its radius. Every other property (diameter, circumference, area) follows directly. Enter any one value and the calculator derives the rest.
π (pi) is an irrational constant approximately equal to 3.14159. The calculator uses the full IEEE 754 double-precision value so results are accurate to at least 15 significant figures before rounding.
Weisstein, Eric W. — Circle. MathWorld, Wolfram Research.Worked example: circle with radius 7 cm
A circle has a radius of 7 cm. Find its diameter, circumference, and area.
Circle measurements for radii 1–10
Use the table below as a quick lookup for common radii. All values use π ≈ 3.14159 and are rounded to two decimal places.
| Radius | Diameter | Circumference | Area |
|---|---|---|---|
| 1 | 2 | 6.28 | 3.14 |
| 2 | 4 | 12.57 | 12.57 |
| 3 | 6 | 18.85 | 28.27 |
| 4 | 8 | 25.13 | 50.27 |
| 5 | 10 | 31.42 | 78.54 |
| 6 | 12 | 37.70 | 113.10 |
| 7 | 14 | 43.98 | 153.94 |
| 8 | 16 | 50.27 | 201.06 |
| 9 | 18 | 56.55 | 254.47 |
| 10 | 20 | 62.83 | 314.16 |
All measurements in the same unit as the radius. Circumference = 2πr; Area = πr².
Tips for circle calculations
Circles appear in construction, engineering, cooking, and design. These tips help you apply the formulas correctly in real-world situations.
- Measure diameter, not radius, in practice — it is easier to measure across a circle than from centre to edge; enter the diameter and let the calculator halve it.
- Area scales with the square of the radius — doubling the radius quadruples the area, so small radius errors grow quickly; measure carefully.
- Circumference for fencing or edging — to border a circular garden bed or pool, use the circumference formula; add 5–10% for overlap and joins.
- Area for coverage problems — paint, turf, or flooring quantities depend on area; divide the area by the product coverage rate to get the amount needed.
- Semi-circles and sectors — halve the area for a semicircle (πr²/2); for a sector, multiply area by (angle/360°) to get the slice area.
Accuracy and limitations
All calculations use JavaScript's built-in Math.PI (accurate to 15+ significant figures). Results are displayed rounded to a configurable number of decimal places. The formulas apply to perfect mathematical circles; real-world objects have manufacturing tolerances and irregular edges that the calculator cannot account for. For engineering or structural work, add an appropriate safety margin beyond the calculated value.
Key terms
About this calculator
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