Circles
The radius goes from the center to the edge of the circle. The diameter passes through the center and is twice the radius.
Diameter = radius × 2
Radius = diameter ÷ 2
Example
Radius 6 cm. Diameter = 6 × 2 = 12 cm
Circumference = 2 × π × radius
Area = π × radius × radius (π = 3.14)
Example
Radius 5 cm. Circumference = 2 × 3.14 × 5 = 31.4 cm