Circle Calculator
Enter a radius to calculate the basic geometry of a circle.
Diameter10
Circumference31.4159
Area78.5398
How this tool works
Diameter d=2r, circumference C=2πr and area A=πr².
Example
A radius of 5 gives diameter 10, circumference about 31.416 and area about 78.540 square units.
Common questions
What units does the calculator use?
Any consistent length unit works; circumference uses that unit and area uses its square.
Can I start from diameter instead?
This version takes radius directly; divide a known diameter by two first.