Online circle basics calculator. Enter the radius to get the diameter, circumference, and area in one go, with formulas—ideal for elementary circle geometry.
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C=2πr, S=πr², d=2r
Center to the circle
📖 Tutorial | Circle
1. Definition
A circle is the set of points at distance r (radius) from a fixed center.
2. Formula & Notation
r, d
Radius, diameter d=2r
C
Circumference, 2πr
S
Area, πr²
3. Properties & Laws
d=2r;
C=2πr=πd;
S=πr²;
π≈3.14159.
4. Steps
Enter radius r;
Click "Calculate";
Or click "Load Example".
5. Worked Examples
Example: radius 5. Solution: C≈31.42, area≈78.54.
6. Common Pitfalls
Area uses r², not 2r; Do not swap C and S; r must be positive.