Circle Calculator
Know any one measurement — radius, diameter, circumference, or area — and get the other three, formulas included.
Diagram
How to use the circle calculator
- Pick the value you know. Radius, diameter, circumference, or area.
- Enter it and press Calculate. The other three appear instantly.
- Read the formulas. Each result carries the exact formula used — handy for showing your work.
- Check the diagram. The radius is drawn and labeled on the circle.
Frequently asked questions
Which formulas does it use?
The four linked relations: d = 2r, C = 2πr (equivalently πd), and A = πr² — rearranged depending on which value you enter. Each result shows the exact formula used.
I only know the area — can it find the radius?
Yes. From the area, r = √(A/π); the diameter and circumference follow from there.
What is the difference between circumference and area?
Circumference is the distance around the circle — a length. Area is the space inside the circle, measured in square units.
How precise is π?
The tool uses full double-precision π internally; results are rounded to four decimals for display.
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Open tool →Support the codex
This tool is free, with no account and no ads. If it saved you some time, consider supporting the indie dev behind it.