Circle area and circumference from radius or diameter.
How to use
- Pick radius or diameter mode.
- Enter one positive length.
Example
Radius 2 → area ≈ 12.57, circumference ≈ 12.57.
More shapes: area calculator.
Frequently asked questions
Area and circumference?
Area πr², circumference 2πr; from diameter, r = d/2.
Radius vs diameter mode?
Pick the mode that matches what the problem gives.
Which π?
Uses full-precision π internally—may differ slightly from 3.14 hand work.
Can I save my inputs?
Yes. Copy link below the form keeps your current inputs so you can share the scenario or save it for later.