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Drag the radius of this circle to resize it and watch what happens to the circle's circumference (called perimeter in this example). If you have trouble with this virtual manipulative, try opening it in "full screen" mode.
The circumference of a circle is not equal to the length of two diameters.
A circle's circumference divided by its diameter is the same for every circle. (It's pi, or 3.14)
This online lesson from onlinemathlearning.com provides a good explanation about how to use either radius or diameter to calculate the circumference of a circle. It includes a couple of embedded videos.