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?

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.

An earned run is any run that is scored by the opposing team, except for runs that are scored due to an error by a defensive player. Innings pitched refers to the number of innings a pitcher has thrown, including any fractional innings. For example, if a pitcher throws 2.1 innings, that counts as 2.1 innings pitched.

The lower a pitcher's ERA, the better their performance is considered to be. For example, a pitcher with an ERA of 3.00 is considered to be performing better than a pitcher with an ERA of 4.00.

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 several factors that can affect a pitcher's ERA. These include the quality of the pitcher's pitches, the skill of the opposing team's batters, and the defensive ability of the team's fielders. A pitcher's ERA can also be affected by the quality of the ballparks in which they pitch, as some ballparks may be more conducive to scoring runs than others.

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

The concept of ERA has its roots in the early history of baseball, when pitchers were responsible for the majority of the runs that were scored in a game. In the early days of the sport, pitchers were expected to throw most of the innings in a game, and their effectiveness was often measured by the number of runs they allowed.

The first known use of ERA as a statistical measure in baseball dates back to the late 19th century. It was originally calculated by dividing the total number of runs allowed by the number of innings pitched, without regard to whether the runs were earned or unearned.

Over time, the definition of ERA has evolved to include the concept of earned runs, which are runs that are scored by the opposing team due to the pitcher's own actions, rather than due to errors by the defensive team. This distinction was introduced in order to provide a more accurate measure of a pitcher's effectiveness, as unearned runs were seen as being beyond the pitcher's control.

Today, ERA is widely used as a key statistical measure in baseball, and is a key factor in determining the performance of pitchers at all levels of the sport. It is used to compare the effectiveness of different pitchers, and to assess the overall quality of a team's pitching staff. So, the origin of ERA is closely related to the early history of baseball and the evolution of statistical analysis in the sport.


Earned run average is a statistical measure that is used to evaluate the performance of pitchers in baseball. It is calculated by dividing the number of earned runs allowed by the number of innings pitched, and is used to compare the effectiveness of different pitchers. By considering a combination of statistical measures, analysts can get a more complete picture of a pitcher's performance.