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As of 2008 the circumference of a regulation size baseball is between nine and nine and a quarter inches which converts to between about 7.3 cm and 7.5 cm in diameter according to the official rules of Major League Baseball (MLB).
In some 12-year-old-and-under youth leagues (such as Little League baseball), the bat may not be more than 2 1â4 inches (57 mm) in diameter. However, in many other leagues (like PONY League Baseball, and Cal Ripken League Baseball), the bat may not be more than 2 3â4 inches (70 mm) in diameter.