Baseball Reference Glossary

Earned Run Average (ERA)

What is Earned Run Average?

The definition of earned run average (ERA) is “the number of earned runs a pitcher allows per nine innings.” Earned runs are runs that are scored because of pitching, not bad defense like errors or passed balls.

Most pitchers do not play nine innings in each game. Thus, the earned run average formula takes the number of earned runs allowed in the innings a pitcher did pitch and uses them to calculate how many earned runs he would have allowed, had he pitched nine innings.

How is ERA used?

A pitcher’s ERA is a primary measure of his success. A low ERA indicates that a pitcher is good at limiting his opponent’s scoring. 

Earned run average is particularly useful for evaluating starting pitchers. To earn a perfect 0.00 ERA, a pitcher must not allow any earned runs to be scored. Additionally, if a pitcher allows a hitter to reach base, and then a different pitcher allows him to score, then the pitcher who allowed the runner to reach is charged with the run.

ERA is similar, but not identical to, run average.

Earned Run Average Formula

The earned run average formula is:

ERA = (Earned Runs Allowed / Innings Pitched) * 9

How to calculate Earned Run Average?

Dividing earned runs a pitcher allowed by the number of innings he pitched gives us the average number of earned runs he allowed per inning. Multiplying that by nine (for nine innings) gives us the number of earned runs he was expected to give up over nine innings.

How to calculate the Earned Run Average for one game?

Say a pitcher allows 3 earned runs over 6 innings. Here’s how you would calculate his ERA:

  1. Divide his earned runs allowed (3) by the innings he pitched (6). 3/6 = 0.5.
  2. Multiply 0.5 by 9. 0.5*9 = 4.5.

The pitcher’s earned run average for that game would be 4.50.

For a real-life example, let’s look at Paul Skenes’ first career start, where he pitched 4 innings and allowed 3 earned runs. Here’s how you would calculate his ERA:

  1. Divide Skenes’ earned runs allowed (3) by the innings he pitched (4). 3/4 = 0.75.
  2. Multiply 0.75 by 9. 0.75*9 = 6.75.

So Paul Skenes recorded a 6.75 ERA in his first career start.

How to calculate the Earned Run Average for multiple games?

We can also calculate a pitcher’s ERA over multiple games, seasons, or even his entire career. This uses the same earned run average formula that we use to calculate single games.

For example, Cy Young pitched 7356 innings and allowed 2147 earned runs. Here’s how you would calculate his career ERA:

  1. Divide his career earned runs allowed (2147) by the number of innings he pitched over his career (7356). 2147/7356 = 0.292.
  2. Multiple 0.292 by 9. 0.292*9 = 2.628.
  3. Round 2.628 to 2.63, since ERA typically only includes 2 decimal places.

Cy Young’s career ERA is 2.63.

Interesting ERA Stats

You can see the single-season leaders in ERA, or the career leaders in ERA on Baseball Reference.

Filed under Statistics