Circumference to Radius Calculator

Your scenarios

Saved on this device. Select a scenario to restore its measurements and units.

Worked example

Circumference 4π m (about 12.56637061 m) gives radius 2 m.

Method

Rearrange circumference = 2πr: radius = circumference ÷ (2π).

Assumptions and limits

The shape is an ideal circle; radius is the distance from its center to its edge. Measurements must be positive. Entered components are limited to 10¹² and canonical quantities/results to 10¹⁵; very small or large calculations may exceed numerical limits. Results use floating point arithmetic and are rounded for display. This is ideal geometry, not a structural or construction safety check.

Common questions

What should I measure?

Measure the full distance around the ideal circle, not its diameter or a partial arc.

Can I enter different units?

Yes. Select each input unit independently. The calculator converts inputs before applying the formula; choose output units for the answer.

How do I save or share an alternative?

Save up to five scenarios for this calculator on this device. Select one to restore its measurements and units, or clear saved scenarios. Copy link includes your measurements in the URL: share it only with people you intend to see them. Other calculator jobs have separate saved scenarios.

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Elementary formulas are derived algebraically and checked with independent numeric and dimensional tests. Unit definitions use the shared reviewed NIST conversion reference.