The diameter will always be twice the radius. If you know the radius, you can multiply by 2 to get the diameter; if you know the diameter; you can divide by 2 to get the radius. Remember that a line that connects two points on the circle (also known as a chord) but does not pass through the center will not give you the diameter; it will have a shorter distance.
A circle located at point 0 would look like this: ⊙O.
As an example, take the equation x^2 + y^2 = 16.
In the example, note that you can write (x – 0)^2 + (y – 0)^2 = 16. You can see that a = 0 and b = 0, and the center of your circle is therefore at the origin, at point (0, 0).
So, in our example, you have a 16 for r, but there is no square. To get the radius, write r^2 = 16; you can then solve to see that the radius is 4. Now you can write the equation as x^2 + y^2 =4^2.
In the example, you would count 4 in all directions to plot the radius points, since our radius is 4.
