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Area Of A Circle Calculator

Find circle area and related measurements from a known radius, diameter, or circumference.

Area Of A Circle Calculator

Circle area

Select a known measurement and enter its value.

Use one consistent length unit; area is returned in that unit squared.

Formula version 1.0.0

How to use this calculator

  1. Choose radius, diameter, or circumference as the known measurement.
  2. Enter its positive value.
  3. Select Calculate.
  4. Review the derived radius, diameter, circumference, and area.

Symbols and units

SymbolMeaningUnit or domain
rRadiuslength
dDiameterlength
CCircumferencelength
AAreasquare length

Checked examples

Radius known

Inputs: Radius 3

Calculation: A=π×3²; d=2×3; C=2π×3

Result: Area 9π; diameter 6; circumference 6π

All circle measures follow from the radius.

Check: circle-area-core.test.ts

Circumference known

Inputs: Circumference 2π

Calculation: r=C÷(2π)=1; A=πr²

Result: Radius 1; area π

Circumference is first converted to radius.

Check: circle-area-core.test.ts

Assumptions, rounding, and limits

  • The shape is a perfect Euclidean circle.
  • The known measurement is positive.
  • Length units are consistent.
  • Area is reported in squared input units.

Frequently asked questions

Is diameter twice radius?

Yes, d = 2r.

Can area be calculated from circumference?

Yes. The calculator derives r = C/(2π) before applying A = πr².

Does this apply to an ellipse?

No. An ellipse requires two semiaxes and a different formula.

Sources and review

Formula version
circle.area@1.0.0
Formula review
Equatara formula review
Editorial review
Equatara editorial review
Last verified
2026-10-25
Applicability
Ideal Euclidean circle from one positive measure.
Next review
On material formula/source change, or by 2028-10-25.

Found a problem?

Report a formula, result, example, source, or accessibility issue through the calculator correction process. Do not include sensitive input values.