Android app on Google Play

Circle

Circle with radius and diameter
r = Radius, d = Diameter
Drag any red dot.
The values of radius and the diameter are rounded to one decimal place.

The diameter is the distance across the circle, the length of the chord passing through the center point. The diameter is twice the radius:

d=2*r

You can also say that Radius is half the diameter or the distance from the center to the edge.

r=frac{1}{2}*d