As you can see in all twenty trials when I divided the new circumference by the old circumference the ration is .5 or one half. Click here if you would like to go to the spreadsheet file and see the formulas or extend the trials. This file will show you the results of a halved diameter. After observing the changes made in the three spreadsheets it is easy to see that the circumference will increase as the diameter increases and the circumference will decrease as the diameter decreases. This change can be described by looking at the formula for Circumference.
Circumference is equal to pi time diameter.
Pi is a constant and never changes, thus d is the only variable in the formula. If the diameter increases, the circumference must increase. If the diameter decreases, the circumference must decrease.