✓ Effortless Math · Free Tool

Circle Calculator

Enter any one of the radius, diameter, circumference, or area — and instantly get all four, with exact π-multiples, decimal values, a live diagram, and full step-by-step working.

Circumference:  C = 2πr  ·  Area:  A = πr²
I know the known = value

Pick what you already know, then type its value. The value must be greater than 0. Decimals are fine.

Results

Summary
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Radius — r
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Diameter — d = 2r
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Circumference — C = 2πr
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Area — A = πr²
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Diagram

CircleRadiusCenter

Step-by-step (circle formulas)