Add the total amount of games the team won during the year. For example, the team won 80 games.
Add the amount of games played during the course of the season. In the example, the team played 120 games.
Divide the amount of wins by the amount of games played to determine winning percentage. In the example, 80 games divided by 120 games equals a winning percentage of 0.667 or 66.7 percent.