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ERA Calculator (Earned Run Average)

Calcuate the earned run average.
Modify the inputs below to calculate the ERA (earned run average). To enter partial innings, use 0.33 for a third of an inning, and 0.66 for two-thirds, and so forth.
ERA
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What is ERA?

In baseball, the earned run average (ERA) is the measure of a pitcher's average performance over a period of time, typically over the course of a season. The ERA is calculated by taking the number of earned runs allowed by a pitcher and dividing it by the number of innings pitched. For example, if a pitcher allows three earned runs in six innings pitched, their ERA would be 4.50.

The earned run average is an important statistic for pitchers as it can give an indication of how effective they are at preventing runs from being scored against them. A lower ERA indicates that a pitcher is better at preventing runs from being scored and is therefore more valuable to their team.

ERA formula

The formula for finding ERA is:

9×Earned RunsInnings Pitched9 \times \frac{\textrm{Earned Runs}}{\textrm{Innings Pitched}}

For example, a pitcher who allows 5 earned runs in 7 innings pitched would have an ERA of 6.43 (9×579 \times \frac{5}{7}).

If a pitcher exits a game with runners on base, then any earned runs scored by those runners will count against him.

Factors that impact the ERA

There are a number of factors that can affect a pitcher's ERA, such as the quality of their opponents, the ballpark they are pitching in, and their own personal abilities. A pitcher with good control over their pitches will typically have a lower ERA than one who doesn't have good control.

Why is ERA used?

Because the goal of pitching is to prevent runs from scoring, and ERA gives us this, ERA is considered to be an excellent metric for evaluating pitchers. On average, how many runs does a pitcher allow that are his fault in a given game?

Flaws with using ERA as a metric to evaluate pitchers

ERA is not a perfect metrics because many different factors can affect it.

  1. Great defensive plays are discounted: A pitcher with an average defense is at a disadvantage to a pitcher with a great defense.
  2. Difficulty of evaluating ERA across the two leagues in Major League Baseball: The absence of a designated hitter in the National League usually keeps pitchers' ERAs lower.
  3. Different ballparks can impact ERA: Some stadiums are more conducive to run scoring.

The origin of ERA

Henry Chadwick, statistician and writer, invented ERA in the mid-to-late 19th century. At the time, the win-loss record was the primary metric used to evaluate pitchers. Chadwick believed this metric was insufficient, and create ERA.

The earned run average is just one measure of a pitcher's performance and should not be used as the sole determinant of their value to a team. Other important statistics to consider when evaluating pitchers include wins, losses, strikeouts, walks, and saves.