It depends on what you mean by a "7-inch circle".
If, as is normally the case, it is a circle with a 7-inch diameter, then you've got your answer in the question.
If, unusually, you mean a circle with a 7-inch radius, then the diameter is twice that - 14 inches.
If, even more unusually, you mean a circle with a circumference of 7 inches, then the diameter is 7/pi = 2.228 inches.
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