An earned run average, or ERA, is used to determine the average number of earned runs that are scored against the pitcher for every nine innings played. The National League began keeping official earned run average statistics in 1912, and the American League started soon after that. A good ERA today is different from a good ERA in the early 1900s, due to changes in the game. Today, an ERA that is less than 2.00 is exceptional. An ERA that's between 2.00 and 3.00 is held by extremely good pitchers. An ERA that's between 3.00 and 4.00 is still above average. Average is between 4.00 and 5.00, and an ERA over 5.00 is below average.
Step 1
Determine the number of earned runs. An earned run is the pitcher's fault. An unearned run would be the result of a fielding error and is not included. For example, if a pitcher gave up three home runs, but then, later in the inning, a fielder's error caused another run to score, only three of the four runs were earned in that inning.
Step 2
Multiply the number of earned runs by nine. If a pitcher allowed 19 earned runs in 89 innings pitched overall, you multiply 19 times nine to get 171.
Step 3
Divide the result from step 2 by the number of innings pitched. In our example, that's 171 divided by 89 for a result of 1.92. In simple form, the calculation looks like this: Earned runs times nine divided by innings pitched.
Step 4
Calculate the formula the other way around. You can take earned runs, divide by innings pitched, and then multiply the result by nine. In our example, you'd have 19 runs divided by 89 innings for a result of 0.213. You multiply that by nine to get 1.92.
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