ERA (Earned Run Average) Calculator and Explanation



What is Earned Run Average?

Earned Run Average (ERA) is a baseball metric used put a pitcher’s number of runs earned while he was pitching in comparison to the number of innings that have been pitched.
It is intended to be a more accurate representation of a pitcher’s abilities than a win/loss ratio while still putting the statistic in the perspective of a full 9 innings.

How to calculate ERA

In Baseball, earned run average is calculated as follows:
\text{Earned Run Average} = \frac{\text{Earned Runs Allowed}}{\text{Innings in a Game}} \times \text{Innings Pitched}
Each out pitched counts as 1/3 of an inning.
Most professional level games are 9 innings, however softball games are typically 7 innings and little league games are often 6 or 7 innings.
ERA is listed to 2 decimal points.


If a pitcher has pitched 98 innings and 1 out and has allowed 28 Earned Runs, then:
ERA = (28 / (98 + 1/3)) x 9
ERA = (28 / 98.333) * 9
ERA = 0.28475 * 9
ERA = 2.5627
Therefore, the pitcher’s ERA is 2.56

What qualifies as an “earned run”?

An earned run is any run that is scored while the pitcher is on the mound that is no an unearned run. An “unearned run” is considered to happen in situations such as:

  • The batter reaches base on an error
  • The batter reaches base on a passed ball
  • A base-runner scores after the 3rd out except for an error being made (except for catcher’s interference)
  • A batter hits a foul ball that is dropped by a fielder in error, and later scores a run in that inning. In this case “how” the batter reached base is no longer relevant (as they would have been out if not for the fielder’s mistake)
  • A batter reaches base on a fielder’s choice when a runner who reached base on an error is eliminated
  • A runner advances on an error or passed ball and scores on a play that otherwise would not have meant they would have scored

If the run is “unearned” but as a responsibility of the pitcher’s in-fielding, then it is still considered “unearned”.
If there is a question of whether or not the run was “earned” or not, the benefit of the doubt is given in the pitcher’s favor.

What counts as an “inning pitched”?

An inning pitched is each full inning (3 outs) that the pitcher has been on the mound.
For partial innings (for example the pitcher pitched 5 innings and 2 outs) each out is counted as 1/3 of an inning – in this case it would be 5 2/3 innings pitched.


ERA as a measurement of a pitcher’s effectiveness became popular around the 1900s.
For a long time, a pitcher’s effectiveness was determined by their win/loss ratio (primarily because pitchers were expected to pitch a full game).
As relief pitching became more popular and pitchers no longer regularly pitched full games, the concept of a “weighted” run to ratio average became more popular.