Baseball and softball bats are sold according to length in inches and weight in ounces. Much to the consternation of players buying new bats, however, not all bats that weigh the same swing the same. The reason for this has to do with moment of inertia of the bat about a pivot point on the handle, or what the sporting goods industry refers to as "swing weight." A number of recent field studies have confirmed that the speed with which a player can swing a baseball or softball bat depends more on the bat's moment of inertia than on its mass. In this paper we investigate the moment of inertia (swing weight) of a variety of baseball and softball bats.