The relationship
Formula
d = 2r · C = 2πr · A = πr²
For a radius of 5 cm: diameter = 10 cm, circumference ≈ 31.4159 cm, and area ≈ 78.5398 cm².
Core geometry
Circle Calculator: Radius, Diameter, Circumference & Area
Enter any one circle measurement. The calculator derives the other three immediately, keeps length and area units clear, and shows the exact relationship behind every result.
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INPUT
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The other measurements update as you type.
RESULT
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Radius5 cm
Diameter10 cm
Circumference31.4159 cm
Area78.5398 cm²
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Common questions
What can I calculate from one known circle measurement?
Any one of radius, diameter, circumference or area is enough to calculate the other three values.
What is the difference between radius and diameter?
The radius runs from the center to the circle edge. The diameter crosses the whole circle through its center and is exactly twice the radius.
Do I need to convert units first?
No conversion is needed when every length uses the same unit. The area result is shown in the matching squared unit.