What's an Earned Run Average?
A major league pitcher is often judged on the basis of his earned run average, or ERA. This number represents the average number of earned runs given up by the pitcher per nine innings.
An earned run is any run that the opponent scores off a particular pitcher except for runs scored as a result of errors. For instance, if Tim Lincecum gives up three solo homeruns, and then an error causes another run to score, he is only credited with those first three runs that were "his fault."
The earned run average can be calculated using the following formula:
(Earned Runs/Innings Pitched) x 9
Therefore, if Roy Halladay is charged with 19 earned runs in his first 89 innings pitched, his ERA would be 19 divided by 89, which is .2135, times 9, which is 1.92, a very good number.
(19 runs / 89 innings) x 9 = 1.92
Don't forget the 9 at the end. By calculating runs/innings you have only figured out earned runs per inning, but you must keep in mind that an ERA is actually earned runs per nine innings, since a regulation game is 9 innings. The number, usually represented with two places after the decimal, shows how many runs the pitcher gives up in an average complete game.
Here's one last example: Johan Santana yielded 66 earned runs over 234.33 innings in 2008. What is his ERA? Simple -- divide 66 runs by 234.33 innings and multiply by 9. The correct answer is 2.53.
Earned run average represents the average number of (earned) runs given up per 9 innings pitched.