Over inflating stability balls is the number one cause of ball failure. The Stability Ball Tape Measure is a fast and easy tool to ensure your ball is inflated properly. Simply insert tape into plug at the top of the stability ball and wrap the tape measure around to the bottom of the stability ball. Read the clearly marked sizes to determine the appropriate diameter. Markings are set for 30 cm. to 85 cm., and 80% inflation for each of the sizes.