Radius is the distance from the center of a circle to any point on the circle.
Diameter is the distance across the circle through the center.
Radius = Diameter ÷ 2
Diameter = Radius × 2
If you know the circumference, Radius = Circumference ÷ (2π)
If you know the circumference, Diameter = Circumference ÷ π
If you know the area, Radius = √(Area ÷ π)
If you know the area, Diameter = 2√(Area ÷ π)