How to find radius
From diameter: r = d / 2. From circumference: r = C / (2 * pi). From area: r = sqrt(A / pi). Each formula isolates the radius from a different known measurement.
Radius Calculator
Find the radius of a circle from its diameter, circumference, or area with all other properties included.
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Also in Circles
Geometry
Find the radius from diameter, circumference, or area
The radius calculator finds the radius of a circle when you know the diameter, circumference, or area. All other circle properties are computed alongside the radius.
Limitations
Applies to perfect circles only. Input must be a positive number.
Frequently asked questions
Divide the area by pi, then take the square root: r = sqrt(A / pi).
Yes, by definition. The diameter is a line through the centre from edge to edge; the radius goes from the centre to the edge.
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