The diameter of a circle is the distance from a point on the circle to a point 
radians away, and is the maximum
distance from one point on a circle to another. The diameter of a sphere
is the maximum distance between two antipodal points
on the surface of the sphere.
If 
is the radius
of a circle or sphere, then
. The ratio of the circumference 
of a circle
or great circle of a sphere
to the diameter 
is pi,
See also
Brocard Diameter, Circumference, Circumradius, Circumsphere, Generalized Diameter, Graph Diameter, Pi, Polygon Diameter, Radius, Sphere, Transfinite Diameter Explore this topic in the MathWorld classroom
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Cite this as:
Weisstein, Eric W. "Diameter." From MathWorld--A Wolfram Resource. https://mathworld.wolfram.com/Diameter.html