Math

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Circumference to Radius Calculator.

Convert a circle circumference to its radius using r = C / (2π).

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Set your values

Results update as you type.

Radius: 4.999996 units

Radius

4.999996units

Results update automatically as you type.

Use Cases

Geometry homework and math practice

Quickly check your answers when solving circle problems that require finding the radius from a given circumference.

Example: If a circle's circumference is 25.13 cm, the radius is 4 cm.

DIY and crafting projects

Determine the radius needed to draw a circle with a specific circumference, such as for cutting fabric or creating circular patterns.

Example: For a circular tablecloth with a circumference of 157 inches, the radius is 25 inches.

Frequently Asked Questions

How do I convert circumference to radius?
Divide the circumference by 2π (approximately 6.2832). For example, if the circumference is 31.4 units, the radius is 31.4 / (2π) ≈ 5 units.
What units can I use for the circumference?
You can use any unit of length, such as inches, feet, centimeters, meters, or miles. The radius will be in the same unit as the circumference you enter.
Why is the formula r = C / (2π)?
The circumference of a circle is C = 2πr. Solving for r gives r = C / (2π). This is a direct mathematical relationship.

Tips & Common Mistakes

Tips

  • Use the same unit for circumference and radius; the calculator outputs the radius in the same unit you enter.
  • Remember that π is approximately 3.14159, so 2π is about 6.28318.
  • If you have the diameter, divide it by 2 to get the radius, but this calculator directly uses circumference.
  • Double-check your circumference value for accuracy, as a small error can affect the radius.

Common Mistakes to Avoid

  • Forgetting to divide by 2π and instead dividing by π only, which gives the diameter, not the radius.
  • Using different units for circumference and radius without converting, leading to incorrect results.
  • Rounding π too early in manual calculations, which can cause slight inaccuracies.

Last updated: August 13, 2026