The radius is the distance from the edge to the center. Diameter is distance from one edge through the center to the other edge. The diameter is twice the radius. If the radius is 1, then twice that is 2.
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The radius of a circle is half the diameter, so the radius of a 6 inch diameter circle is 3 in.
d = 2r, where d is diameter and r is radius.
2 inches
6 inches
10.5 inches