In classical geometry, the "Radius" of a circle or sphere is any line segment from its center to its perimeter the radius of a circle or sphere is the length of any such segment.
Applicable units are those of quantitykind:Length
http://en.wikipedia.org/wiki/Radius
http://dbpedia.org/resource/Radius
In classical geometry, the "Radius" of a circle or sphere is any line segment from its center to its perimeter the radius of a circle or sphere is the length of any such segment.
$r = \frac{d}{2}$, where $d$ is the circle diameter.
http://www.iso.org/iso/catalogue_detail?csnumber=43012