Formula for the diameter of a circle depends on what is given to you.

1) Given: Radius of a circle

Then, Diameter  =  Radius  ×  2



2) Given: Circumference of a circle

Then, Diameter  =  Circumference  ÷  π

3) Given: Area of a circle

Then, Diameter  =  2 ×  √(Area  ÷  π)