(Check out the rest of our baseball glossary here.)

Q: What Is ERA in Baseball?

ERA stands for Earned Run Average.

It is a pitching statistic designed to measure the individual effectiveness of a pitcher.

Earned Run Average is calculated as follows:

ERA = 9 * Number of Earned Runs (ER) / Number of Innings Pitched (IP)

The rule regarding what constitutes an earned run — as opposed to an unearned run — is fairly complicated, but the gist is that an earned run is one that is solely attributable to the actions of the pitcher himself.

As MLB.com puts it:

An earned run is any run that scores against a pitcher without the benefit of an error or a passed ball.

The metric gives a rough estimate of how many runs you might expect a pitcher to give up over a complete game (9 innings, hence the multiplier above) during a particular time period, usually a season or season-in-progress.

The history of ERA stretches all the way back to the 19th Century when writer and statistician Henry Chadwick began to suspect that win-loss record was not a full indicator of a pitcher’s effectiveness.

Chadwick’s argument was that simple winning percentage relied too much on factors beyond a hurler’s control — such as the team’s offensive output and fielding prowess — to be used as fair judgment of a pitcher’s value.

Chadwick developed ERA to get a closer read on a pitcher’s true value to his team.

Earned Run Average has been the target of heavy criticism during the Sabermetrics era because it still folds in a good helping of contributions from other players and even ballpark factors.

Even so, ERA remains a widely reported statistic and is at least a decent heuristic when grouping or evaluating pitchers.

(Check out the rest of our baseball glossary here.)

Want a done-for-you ERA calculation? Check out our ERA calculator right here.

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