Math & Statistics Calculators · Geometry
Free Circle Calculator for Radius, Diameter, Area, Circumference, Arcs, Sectors, and Chords
Enter one core circle measurement to solve the others, then optionally add a central angle to calculate arc length, sector area, chord length, and a point on the circle.
Browser-basedNo sign-upPrivate local historyStep-by-step
Interactive calculator
Radius—
Circumference—
Area—
Enter one circle measure.
How to use it
- Choose which circle measurement you already know.
- Enter its value using a consistent length or area unit.
- Optionally enter a central angle from 0° to 360°.
- Calculate all equivalent circle measurements and sector values.
- Remember that area units are squared while radius, arc, and circumference are linear.
What this calculator includes
1. Solve from radiusUse radius as the base measurement.
2. Solve from diameterConvert diameter to radius automatically.
3. Solve from circumferenceRecover radius from C = 2πr.
4. Solve from areaRecover radius from A = πr².
5. Arc lengthCalculate an arc from a central angle.
6. Sector areaCalculate the area of a circular sector.
7. Chord lengthCompute the chord subtended by the angle.
8. Point coordinatesCalculate x,y on a circle at the selected angle.
9. Formula stepsShow substitutions using π.
10. Unit-neutralWorks with any consistent length unit.
11. Local historySave recent geometry calculations.
12. Visual sketchDraw the circle and selected radial angle.
Result checklist
0 / 5 complete
Local calculation history
Formula and reference table
| Quantity | Formula | Units |
|---|---|---|
| Diameter | d = 2r | length |
| Circumference | C = 2πr | length |
| Area | A = πr² | length² |
| Arc length | s = rθ (θ in radians) | length |
| Sector area | Asector = θr²/2 | length² |
Quick knowledge check
If a circle radius doubles, what happens to its area?
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Frequently asked questions
Can I calculate a circle from area only?
Yes. The radius is the square root of area divided by π.
What units should I use?
Any consistent linear unit works. Area is returned in the corresponding squared unit.
How is arc length calculated?
The central angle is converted to radians and multiplied by radius.
What is a chord?
A chord is a straight line segment connecting two points on a circle.
Scope and accuracy
This calculator assumes ideal Euclidean circle geometry and browser floating-point precision. Measurements from physical objects may introduce measurement uncertainty not represented here.