You use the information that you do have, and a formula that relates it to the
radius or diameter of the circle. As an example: The circumference is very helpful
if you know it.
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diameter = 2 * radius
Double the Radius to Calculate the Diameter.
to calculate circumference you find a circles radius or diameter. you do diameter times pi or radius times 2 times pi. the you have circumference!
You can either measure it, or calculate it if you know the diameter (radius = diameter / 2), or the circumference (radius = circumference / (2pi)).
If you know the circumference, the ration of the diameter to the circumference is π, so diameter = circumference/π If you have the radius, diameter = 2 x radius