A 55cms diameter inflatable gym ball suitable for many types of exercise, yoga & pilates. Helps Improve Balance, Coordination and Flexibility. Ideal for Core Strength Training, Stretching, Toning, Resistance Exercises. Question: Ball size is smaller than advertised? Answer: Ball is made of rubber which will become much contracted being packed tightly, more so if temperature is cold. Thus, keep pumping to stretch the rubber is the key. If it is hard to reach suggested ball size during 1st inflation, users can leave the ball as it is for a day, and then pump again after the PVC material gets fully stretched. Also, this is easier in a warm environment.