Quick start
- Choose radius, diameter, circumference, or area mode.
- Enter the known value.
- Add a unit label if useful.
- Press Calculate circle to find radius, diameter, circumference, and area.
Circle formulas used
The calculator uses d = 2r, C = 2pi r, and A = pi r^2.
When you start from diameter, circumference, or area, it rearranges the formula to find radius first.
Choosing a starting value
Use radius mode when you know the distance from the center to the edge.
Use diameter mode when you know the full width through the center. Use circumference or area mode when those measurements are already known.
Reading units
Radius, diameter, and circumference use length units. Area uses square units.
If you enter cm as the unit label, the area result is shown as cm^2.
Examples from the calculator
Area = 78.5398163397
Radius = 5
Radius is about 5
History, privacy, and copying
Recent answers stay visible in the page while you work. The history is kept only in the current browser tab and is not sent to a server.
Copy answer copies the expression and result so you can paste it into notes, homework, a message, or another document.