Earned run average is the most mainstream measure of a pitcher’s performance in baseball. Earned run average extrapolates pitchers’ statistics to determine how many runs they give up over the course of an average nine inning game. As a reference point, in Major League Baseball, the average ERA is around 4.50 with the best pitchers usually sitting between 2.50 and 3.00 (with variations between the two leagues – NL pitchers will generally have lower ERA numbers).
The only two base statistics needed to calculate earned run average are earned runs and innings pitched. Using total runs would be inaccurate because this includes runs scored as a result of fielding errors and passed balls. Earned run average is meant to express pitching effectiveness alone as a statistic.
Convert any partial innings pitched from a fraction to a decimal, treating each individual out as .333 innings. This is important because when written in the box scores or statistics databases, a pitcher’s innings are usually only written as .1 per out, such that five innings and one out would be written as “5.1” innings, which is inaccurate from a mathematical standpoint. As such, treat five innings and one out as 5.333 innings, or five innings and two outs as 5.667 innings.
Now divide the total number of earned runs by the converted innings total (earned runs / innings pitched). If a pitcher relinquishes two earned runs in five and one-third innings, divide 2 by 5.333, which in this example equals 0.375. This number effectively equals earned runs allowed per inning pitched.
Finally, multiply the earned runs per inning total by nine. Nine represents the total number of innings in a standard game. In the example of a pitcher who allows two runs in five and one third innings, this equals 3.375 (0.375 x 9). ERA is generally measured to two decimal points, so rounding this number leaves a final ERA of 3.38.
The full equation will look like this: (earned runs / innings pitched) x 9. Simply plug in your earned runs and innings values to complete the calculation.
Earned run average is not an ideal measure of a pitcher’s effectiveness. It does not take into account the factor that defense plays, a pitcher’s ability to eat innings, or any park factors. Still, it is simple and straightforward, and it is the most frequently used statistic for a pitcher’s effectiveness. It is best used when examined alongside strikeout rates, walk rates, groundball to flyball ratios, and innings totals.