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How to calculate the diameter of a circle 
If you know the radius of the circle, double it to get the diameter. 
The radius is the distance from the center of the circle to its edge. For example, if the radius of the circle is 4 cm, then the diameter of the circle is 4 cm x 2, or 8 cm.
If you know the circumference of the circle, divide it by π to get the diameter. Π is equal to approximately 3.14 but you should use your calculator to get the most accurate results.