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Math

Circle Radius Calculator.

Calculates the radius of a circle from its diameter, circumference, or area.

Results update live as you type.
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Your inputs

How it works

  1. 1

    Enter any one of the three measurements: diameter, circumference, or area.

  2. 2

    The calculator computes the radius from each provided value.

  3. 3

    If multiple values are entered, it returns the largest radius among them.

  4. 4

    The result is displayed with two decimal places.

max(diameter/2, circumference/(2*pi), sqrt(area/pi))

Frequently asked questions

What if I enter more than one measurement?

The calculator uses the largest radius computed from any entered value, so you can check consistency.

Can I use different units for each input?

No, all inputs should be in the same unit system. The result will be in that same unit.

What if I enter zero for all fields?

The radius will be zero, which is correct for a point circle.

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Results

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Radius

5units

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Estimate for general guidance only — verify important decisions with an appropriate professional.

How it works

Calculates the radius of a circle from its diameter, circumference, or area.

  1. Enter any one of the three measurements: diameter, circumference, or area.
  2. The calculator computes the radius from each provided value.
  3. If multiple values are entered, it returns the largest radius among them.
  4. The result is displayed with two decimal places.

Formulas

The math behind this calculator, written out so you can verify the result.

From diameter

r = d / 2

The radius is half the diameter.

Example:

Input: d = 10

Calculation: 10 / 2

Result: 5

From circumference

r = C / (2π)

The circumference is 2π times the radius, so divide by 2π.

Example:

Input: C = 31.4159

Calculation: 31.4159 / (2 × 3.14159)

Result: ≈ 5

Real-world use cases

Where this calculation shows up in everyday life.

Geometry homework

Quickly find the radius when given any circle measurement.

Example: Given a circle with circumference 25.13 cm, find the radius.

DIY projects

Determine the radius of a circular table or garden bed from its diameter or area.

Example: A circular garden has an area of 50 m²; what is its radius?

Engineering calculations

Convert between circle properties for design specifications.

Example: A pipe has a diameter of 0.5 m; what is its radius?

Tips and common mistakes

Tips

  • Use consistent units for all inputs.
  • If you only know one measurement, leave the others at zero.
  • For a quick estimate, remember that circumference ≈ 6.28 × radius.
  • Double-check your area value: it should be in square units.

Common Mistakes to Avoid

  • Forgetting to divide the diameter by 2.
  • Using the diameter instead of the radius in the circumference formula.
  • Mixing units (e.g., meters for diameter and centimeters for area).

Assumptions and limitations

  • Use the stated inputs and units.
  • Results are estimates for planning and education.
  • Check measurements and source data before making an important decision.