Earned Run Average. A baseball statistic defined as the average number of earned runs surrendered by a pitcher per nine innings pitched. The formula is:
ERA = ER * 9 / IP
Where ER is the number of earned runs and IP is the number of innings pitched. The right side of the equation has been simplified from "ER / (IP / 9)".
In Major League Baseball, ERA is represented with two fixed decimal places. An ERA under 3.00 is considered to be very good, while an ERA under 2.00 is outstanding to maintain through a season. This stat can also become very inflated early in the season; if a pitcher surrenders one run in one-third of an inning, his ERA is therefore (1 * 9 / (1/3)) = 27.00. Because a pitcher can surrender runs without getting an out (thus pitching 0 innings) this figure can also be infinite.