From BR Bullpen
The Magic number is a number used to indicate how close a team is to clinching a pennant, a division title or a spot in the post-season. It represents the total of additional wins by that team or additional losses by its closest pursuer required to make it impossible for that team to be caught before the season ends. Fans start talking about the magic number in early September, when a team has a significant lead and the possibility of its being caught becomes more remote. The number can then be counted down with each win (or loss by the closest pursuant) until it reaches 0 and the team can celebrate its title.
The number is calculated by adding the total number of games remaining in the season, adding one, and then subtracting from this the number of the leading team's wins and the pursuant team's losses:
- G : number of games during the season
- WA : number of wins by the leading team
- LB : number of losses by the pursuing team
There are various other formulas to calculate the magic number, but all come up with the same result. Thus, in a 162-game season, each team starts with a magic number of 163, which is why once in a while a fan will jokingly hold a poster in the early days of a season, when his favorite team is on a hot streak, reading something like: "Magic number 135". A magic number can be calculated for each team behind the leader. Once the number for that team becomes 0, that team is said to be "mathematically eliminated".