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How to Calculate ERA

How to Calculate ERA
by Adam of

The Basics of ERA

Earned run average is one of the most widely used barometers for a pitcher's performance. It is simply the average amount of earned runs a pitcher allows for every 9 innings he pitches. Thus, you want this number to be as low as possible. Good relief pitchers have ERA's that typically fall below 3.00, whereas anything below 3.50 is good for starting pitchers.

The earned run statistic is therefore very important to understand, and often a source of confusion to even the most seasoned baseball enthusiasts. An earned run is most easily understood in my opinion by knowing what an unearned run is... and an earned run is simply anything that isn't unearned. In addition, a run is ALWAYS attributed to the pitcher who put him on base (not the pitcher on the mound when he scores).

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An unearned run occurs when any of the following SCORES:

1. A batter who reached base on an error.

2. A runner who stayed on base because of an error that would have been a fielder's choice, and thrown him out.

3. A batter who reaches base on a passed ball (for strike three).

4. Any runners who scored after the third out should have been made (an error prolonged the inning).

5. A batter who reached on a fielder's choice, where the lead runner would have been an unearned run.

6. A runner scores because they were able to advance an extra base via an error. Otherwise that runner would not have scored.

Earned Run Average: Calculation

To calculate earned run average, you only need the number of innings pitched and earned runs allowed by a pitcher. Going back to math class, we use fractions to see and calculate the relationship between these numbers and the statistic that is earned run average.


Earned runs allowed / innings pitched EQUALS Earned run average / 9.

In other words, you are simply calculating the equivalent fraction of earned runs allowed over the number of innings pitched to the number of earned runs that equals over the course of only 9 innings.

Simple calculation:

To solve the above formula, you solve the calculation for ERA as follows:

Earned runs allowed MULTIPLIED BY Nine DIVIDED BY innings pitched.

Examples of ERA

To illustrate the earned run average calculation, assume we have these statistics:

Innings pitched: 150

Runs Allowed: 54

Earned Runs Allowed: 50

To solve it, note that we only use earned runs allowed as opposed to runs allowed. We also round off the earned runs average statistic to two decimal places:

50 x 9 / 150 = 450 / 150 = 3.33333333 = 3.33

ERA: Uses and Implications

As are all stats, ERA is a useful metric, but not all inclusive. That is, looking at ERA alone does not tell you everything you need to know about a pitcher. Some pitchers are better than their ERA implies, whereas others are not as good as their ERA might lead you to believe.

To understand ERA's importance as well as shortcomings, let us look at the nature of the statistic. It takes into account only earned runs as opposed to all runs, normalized to a span of 9 innings. This is helpful because it enables you to compare pitchers of all types. It also does not count for unearned runs which are often not the fault of the pitcher.

On the other hand, inherited runners who score are credited as runs against the pitcher who put that runner on base. This is misleading and sometimes inaccurate because the pitcher who allowed the runner to score may be more "at fault" than the pitcher who put the runner on; yet, the earned run averages will reflect the opposite. For instance, assume PITCHER A is facing BATTER 1. PITCHER A is a tired pitcher ready to come out of the game. The manager decides it would be best to intentionally walk the current batter. PITCHER A does that. Next, PITCHER B comes on to pitch in relief. The next batter, BATTER 2, hits a homerun. The two earned runs are split even though PITCHER B is clearly to blame. BATTER 1's earned run goes to PITCHER A, while BATTER 2's earned run goes to PITCHER B. So, you can see how a statistic like ERA is better served at times in conjunction with other stats. In this case, and for all relief pitchers, it helps to look at stats involving the percentage of inherited runners a relief pitcher allows to score.

It is also better to look at ERA as a relative statistic. Pitchers pitch in different ballparks, leagues, etc. For instance, Coors Field is a hitter's park. Very rarely will you see a Colorado Rockies pitcher with a stellar ERA. He is at a distinct disadvantage in his home park, of which he plays half of his games. So, it helps to look at these pitchers' away vs. home ERA splits, as well as their ERA compared to teammates who pitch under the same circumstances, to see how they truly stack up against other pitchers. Therefore, it is very important to understand ERA, and maybe even more important to understand that it is not the only statistic we can use.

Good luck and as always, thanks for reading!

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