In classical geometry, a radius of a circle or sphere is any line segment from its center to its boundary. By extension, the radius of a circle or sphere is the length of any such segment. The radius is half the diameter. In science and engineering the term radius of curvature is commonly used as a synonym for radius.

More generally—in geometry, engineering, graph theory, and many other contexts—the radius of something (e.g., a cylinder, a polygon, a graph, or a mechanical part) is the distance from its center or axis of symmetry to its outermost points. In this case, the radius may be more than half the diameter.

The relationship between the radius and the circumference of a circle is $r\; =\; \backslash frac\{c\}\{2\backslash pi\}$.

See also

• Radius of curvature
• Radius of curvature (optics)
• Bend radius
• Curvature
• Diameter

Geometry | Length

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