What Is a Pitcher's ERA?

In the sport of baseball the term "ERA" refers to a pitcher's earned run average. This is an important statistic for pitchers, as it is a reliable gauge of how effective they are against the batters that they face. Earned run average is not difficult to calculate and some of the lowest earned run averages for a career and for a single season belong to Hall of Fame hurlers.

Earned runs

To understand an earned run average, it is important to first grasp the concept of earned runs. These are runs that the game's official scorer decides are the by-product of an offensive side's prowess and not the result of defensive errors or mistakes. When teams score runs via base hits and walks without the help of errors or passed balls, these runs are always "earned."

Calculating ERA

To calculate a pitcher's earned run average, the statistician takes the number of earned runs he has allowed and multiplies this number by nine. This total is divided by the number of innings the pitcher has pitched. For instance, a pitcher that has allowed 36 earned runs over 144 innings will have an earned run average of 2.25 based on the following calculations: 36 earned runs multiplied by 9 is 324. 324 divided by 144 innings pitched yields an ERA of 2.25.

Relief pitchers

When a pitcher leaves a game in the middle of an inning in favor of a relief hurler, any runners that are currently on base will be his or her responsibility. If these runs score they will be charged to the previous pitcher and not the reliever, even if the reliever is the one that allows them to score by giving up hits to the opposition.

Comparing ERAs

Major League Baseball earned run averages over the years are difficult to compare to each other. In the early days of baseball, the pitcher was not 60.5 feet from home plate as is now the case, so earned run averages from these distances cannot be used to compare pitchers. The ball was not as lively before 1920--even when the mound was moved back to 60.5 feet--so earned run averages from that time would be lower. Indeed, the all-time record for the lowest ERA for an entire career belongs to Ed Walsh, who pitched mostly for the Chicago White Sox from 1904 through 1917.

Single season ERAs

Of the top 50 best single season earned run averages posted by a pitcher, only three occurred after 1920 when the livelier ball was introduced into play to boost offense and attendance at Major League Baseball games. Greg Maddux had an ERA of 1.56 in 1994 for the Braves, while Dwight Gooden had one of 1.53 in 1985 for the Mets. Bob Gibson of the Cardinals had what many consider the greatest year ever for a pitcher in terms of earned run average. In 1968, Gibson allowed only 38 earned runs in 304 innings for an earned run average of 1.12.