The circumference would also be doubled. The formula to find circumference is 2π r; since radius is just the diameter divided by two, if you double it the radius also doubles. All in all, this doubles the circumference. For example, if the diameter was 10, the radius would be 5. Circumference for this circle would equal 31.4. Now, if you were to double the diameter to equal 20, thus making the radius 10, the circumference would be 62.8.
