Circumfrence is to a circle as perimeter is to a polygon. The circumfrence of a circle always equals the diameter (or two times the radius, since the radius is half of the diameter) times pi.
The circumfrence would be 7926.41π miles because of this.
Should you round pi to 3.14, the circumfrence would be 7926.41 * 3.14.
This is equal to 24888.9274 miles. (round that if needed)