Roster expansion

Roster expansion refers to a period when teams can carry more than the usual maximum of 25 players on their active rosters (or 26 starting in 2020).

Since 1969, the period of roster expansion has run from September 1st to the end of the regular season (the regular rosters return for the postseason, but not for any one-game playoffs, which are considered to be part of the regular season). Before 1968, there was also a period of roster expansion at the start of the season, in April, when teams could carry a few extra players before making their final roster cuts on May 15th.

Theoretically, a team could have everyone on its 40-man roster eligible for games during the roster expansion period. However, in practice, such a number would be unwieldy, and would carry significant costs. In practice most teams expand their rosters to around 30 players. However, in order to reduce disparities among teams, MLB defined roster expansion as 28 players, including a maximum of 14 pitchers, starting in 2020. This was in tandem with the increase of the active roster to 26 players, with a maximum of 13 pitchers.

There have also been other specified periods when rosters have been set at more than 25 players, such as after strikes in 1990 and 1995, or, more recently, during doubleheaders (the so-called 26th man rule, which became the "27th man rule"). However, these were considered to be specific exceptions to the usual rules, and not periods of roster expansion. This is mainly significant in calculating a player's eligibility as a rookie.