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.

What is ERA?

ERA, or earned run average, is the number of earned runs a baseball pitcher allows per nine innings. Earned runs are runs that scored without the aid of an error or a passed ball. ERA is a commonly accepted statistic for evaluating pitchers.

A lower ERA is better than a higher ERA. The lowest possible ERA is 0, which means that there were 0 earned runs.

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.

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.