The BOSU Ballast Ball comes in two sizes - 45 cm and 65 cm. The Ballast Ball is like no other exercise ball in the gym. This ball has a six-sided surface design for proper alignment and positioning. It also has 2.5 lbs. of multi-dimensional load (ballast) which keeps it from wandering during your workout. The Ballast Ball provides a fun, visual effect and audible feedback during lifting, shifting and shaking exercises. With the Ballast Ball, individuals can perform exercise progressions and dynamic drills that are not possible on a regular exercise ball.