This article relies largely or entirely on a single source. (July 2013)
In most North American sports, the phrase games behind or games back (often abbreviated GB), is a common way to reflect the gap between a leading team and another team in a sports league, conference, or division. It is calculated using the following formula, in which TeamA is a leading team, and TeamB is a trailing team:
This formula gives the GB for TeamB. (One also says that TeamA is that many games ahead.) Thus if Team A has a record of 20 wins and 15 losses, while Team B has a record of 17 wins and 19 losses, Team A would be +5 (i.e., 20-15) while Team B would be -2. Since the difference between those two numbers is 7, Team B is half of that amount --- i.e., 3½ games --- behind Team A, and Team A is that same 3½ games ahead of Team B.
The formula accounts for the difference in the number of games played by the teams by assuming that a team has a 50/50 chance of winning (or losing) the difference in the number of games played. I.e., in the above example, Team A has played 35 games and Team B has played 36. If team A played one more game to match the total games played by team B and has a 50/50 chance of winning, their predicted record would be 20.5 wins and 15.5 losses, hence they are 3.5 games ahead of Team B (20.5-17 = 3.5 wins better and 19-15.5 = 3.5 losses better). The formula above is a simplified version of Team A's Wins - ((Team A's Wins + Losses) - (Team B's Wins + Losses))/2 - Team B's Wins.
A leading team itself is always zero games behind, by definition, and this is typically indicated in tables by a dash rather than a zero.
For example, if the Cleveland Indians are three games behind the Kansas City Royals, Cleveland would have to win three games, and Kansas City would have to lose three, to tie for first by a GB calculation. A "games behind" situation changes rapidly when two teams contesting the lead play each other, since in each such game the GB will change by plus or minus one. In this case, Cleveland could pull even with Kansas City by a clean sweep of all games of a three-game series.
Note that a team may be ahead of a team that has more wins. For example, a team with a record of 65 wins and 53 losses is one-half game ahead of a team with a 66–55 record. Also, two teams with different win-loss records may both be leading teams, if one team has the same number of additional wins and additional losses compared to the other.
The GB calculation is often used in professional baseball and basketball, where tie games are not permitted, and to a slightly lesser extent in American football (where ties are rare and are officially count as half a win and half a loss, but can be omitted entirely when calculating GB since the "wins minus losses" values for a given team will be the same either way) and in ice hockey (where standings points are used). Usually, in tables of standings in newspapers and magazines kept during the season, teams are ordered by the teams' winning percentages, with the number of games they are behind the division leader supplementing the table. On rare occasions, it is possible (and has occurred) for the team with the second best winning percentage to lead the team with the best winning percentage in terms of games back. This usually occurs during the early portion of a season when teams have played an unequal number of games. For example, if Team A has a record of 23–13 and Team B has a record of 26–15, Team A would be recognized as having a better record by virtue of their .639 win percentage to Team B's .634 percentage, and would be listed first in the standings. However, Team A would still be one-half game behind Team B.
The "games behind" number is sometimes made in reference to ".500" (even in wins and losses) although in this particular context, the word "behind" is replaced by "under" or "below". In making this calculation, however, the division by two is not done. For example, a team with a record of 19 wins and 20 losses is considered as being "one game under .500", in contrast to being "one-half game behind" a team with a ".500" record of 20 wins and 20 losses.
Major League Baseball
Major League Baseball defines Games Behind as "the average of the differences between the leading team wins and the trailing team wins, and the leading teams losses and the trailing team losses." 
This statistic is now being calculated somewhat differently in the Major League Baseball professional baseball league. Traditionally games behind was calculated with respect to the team with the highest standing in the league (up through the 1968 season), or to the team with the highest standing in the division (1969 season and later).
The statistic got more complicated for the 1994 season Major League Baseball's two leagues split into three divisions each, and added a wild card team to the playoffs. Following the change, it became common for the media to publish an additional set of standings for the wild card race. It included all teams from a league, with the exception of the division leaders, and games behind was calculated with respect to the team with the highest standing in the wild card race.
The statistic got even more complicated in 2012 when Major League Baseball added a second wild card team. Today, games behind is shown in the media as calculated in two different ways. While games behind the team with the highest standing in the division is calculated in the traditional manner, games behind in the wild card race is calculated with respect to the team with second highest standing in the wild card race. Assuming two or more teams are not tied for first place in the wild card, this results in the team leading the wild card race shown as being so many "games ahead" of second place, usually indicated by a plus sign ("+") in the standings. Currently, both the divisional and wild card statistics are called Games Behind or GB in the media. Major League Baseball's official website distinguishes the two by abbreviating the divisional statistic and the wild card statistic as "GB" and "WCGB" respectively.
National Basketball Association standings typically report the number of games each team is behind a division leader, although it is not as closely followed in basketball because many more teams qualify for the playoffs in the NBA. Unless one particular division is so weak that only the division winner is likely to make the playoffs, the divisional GB statistic is not important from a playoff qualification standpoint. Sometimes, especially nearing the end of the regular season, GB will be given with respect to the #8 position in the conference standings rather than the #1 spot. Since the 8th position is the last qualifier for the playoffs, this stat gives the GB for teams seeking a playoff berth.
National Football League standings also sometimes report games behind, although the statistic became somewhat more complicated to calculate after the league began counting tie games in the standings for the 1972 season. Although tie games are relatively uncommon in American football, when they happen they can lead to the GB statistic being reported in increments of a quarter game for teams that have a tie and played a different number of games than the division leader, or have played a different number of games than a division leader that also has a tie. In any event, the GB statistic has never been as heavily emphasized in the NFL since teams play only sixteen games and typically only play once a week as opposed to almost every day like in baseball.
The GB statistic is eschewed in sports where tie games are traditionally common such as soccer and ice hockey. While most hockey competitions including the National Hockey League have replaced tie games with some combination of overtime and shootouts, they continue to count overtime and shootout losses in more or less the same way tie games were previously counted, so the same principle applies.
- Magic number (sports), number of wins needed to clinch a championship