Expert: Tom Schott - 2/22/2009

Question
What is the formula for earned runs average?  And can you explain what that formula gives me in basic terms?  Thanks!

Answer
Elizabeth,

Sure. The ERA is one of the key measures of pitching performance. It is average that is figured on the number of innings a pitcher has thrown and it tells you how many earned runs a given pitcher will surrender over the course of nine innings. An earned run is one that scores as a result of the other team's offense. These runs are the pitcher's fault because it's his job not to allow the opponents to score any runs. Runs that score because of mistakes made by the fielders (errors) are not counted against the pitcher. Those are called unearned runs.

So the lower the number of earned runs a pitcher allows over the course of a season, the lower his ERA. The way it is determined is by taking the number of earned runs a pitcher has allowed and multiplying it by 9 and then dividing the result by the number of innings a pitcher pitched. The formula looks like this: ER*9/IP. The ERA is always rendered as a whole number with two decimal places.

Generally speaking, an ERA of 3.50 or below is good. If a pitcher's ERA is below 3.00, that is very good. Anything below 2.00 is outstanding and quite remarkable and rare in major league baseball.

Here's an example: One of the great pitchers of all time, Walter Johnson, had an ERA for his career of 2.17. Here's how it was figured.
He pitched 5914.7 innings in his career and gave up 1424 earned runs. So applying our formula we get 1,424*9 = 12,816 / 5914.7 = 2.1668 or 2.17.

It works exactly the same way no matter how many innings we're talking about: a month's worth, a season's worth. or a career.