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$\mathrm{ERA} = 9 \cdot \frac{\mathrm{ER}}{\mathrm{IP}}$

In baseball statistics, earned run average (ERA) is the mean of earned runs given up by a pitcher per nine innings pitched. It is determined by multiplying the number of earned runs allowed by nine and dividing by the number of innings pitched. Runners reaching base on errors (even errors by pitchers) do not count toward ERA if they later score.

Henry Chadwick is credited with first devising the statistic, which caught on as a measure of pitching effectiveness after relief pitching came into vogue in the 1900s. Prior to the 1900s—and, in fact, for many years afterward— pitchers were routinely expected to pitch a complete game, and their won-loss record was considered sufficient in determining their effectiveness. After pitchers like James Otis Crandall and Charlie Hall made names for themselves as relief specialists, gauging a pitcher's effectiveness became more difficult using the traditional method of tabulating wins and losses. The National League first kept official earned run average statistics in 1912 (the statistic was called Heydler's Statistic for a while, after then-NL secretary John Heydler), with the American League following suit afterward.

Modern-day baseball encyclopedias notate ERAs for earlier years, but these were computed many years after the actual accomplishments. Negro League pitchers are often rated by RA, or total runs allowed, since the statistics available for Negro League games did not always distinguish between earned and unearned runs.

As with batting average, the value of a good ERA varies from year to year. In the 1910s, a good ERA was below 2.00 (two earned runs allowed per nine innings). In the late 1920s and 1930s, when conditions of the game changed in a way that strongly favored hitters, a good ERA was below 4.00; only a pitcher of the caliber of Dazzy Vance or Lefty Grove would consistently post an ERA under 3.00 during those years. In the 1960s, sub-2.00 ERAs returned as ballparks with different dimensions were introduced, among other influences. Today, an ERA under 4.00 is again considered very good, although pitchers such as Greg Maddux and Pedro Martínez stand out as Grove and Vance did in their day.

The all-time single-season record for lowest ERA in a season is 0.86, set by Tim Keefe in 1880. The modern record is 1.12, set by Bob Gibson in 1968. The lowest single-season ERA of an active pitcher is 1.56, achieved by Greg Maddux in 1994. The career record is 1.82, held by Ed Walsh, and the active player with the lowest career ERA (among those with more than 1,000 innings pitched, a threshold that filters out most relief pitchers) is Martínez, with an ERA of 2.72 through the 2005 season. Mariano Rivera (career ERA of 2.29 through the end of the 2006 season) finished the 2006 season with 881 2/3 innings pitched, and has a strong chance to finish with more than 1,000 innings lifetime, earning the right, in many fans' minds, to be considered on an equal footing with starters in debates involving the term "greatest pitcher".

Some sources may list players with undefined or infinite career ERAs. This can happen if a pitcher allows one or more runs without retiring a batter (usually in a single appearance).

In modern baseball, an ERA under 2.00 is considered exceptional and is rare. An ERA between 2.00 and 3.00 is also considered excellent and is only achieved by the best pitchers in the league. An ERA between 3.00 and 4.00 is above-average. An ERA between 4.00 and 5.00 is average; the majority of pitchers have an ERA in this range. An ERA above 5.00 is generally considered below-average, and a pitcher with an ERA above 6.00 for a prolonged period of time is usually in danger of demotion to the bullpen or a lower league.

It can be misleading to judge relief pitchers solely on their ERAs, because a pitcher is responsible only for the runs scored by batters who reach base off him. If a relief pitcher enters the game with his team leading by one run, 2 outs in the inning, and the bases loaded, then gives up a single which scores two runs, he is not charged with those runs. If he retires the next batter, his ERA for that game would be 0.00 despite having surrendered the lead. In addition, relief pitchers know beforehand that they will only be pitching for a relatively short while, allowing them to throw each pitch with maximum energy, unlike starters who need to keep something in reserve in case they are called upon to pitch 7 or more innings. This freedom to use their maximum energy for a few innings, or even for just a few batters, helps relievers keep their ERAs down.

ERA, taken by itself, can also be misleading for starting pitchers, though not to the extent seen with relief pitchers. Since 1973's advent of the designated hitter rule in the American League, pitchers spending all or most of their careers in the AL have been at a disadvantage in maintaining low ERAs compared to National League pitchers who can often get an easy out facing the ninth batter (oddly, Martinez and Rivera, the ERA kings of the last decade or so, have been mostly active in the American League). This discrepancy between the leagues also affects relievers, but not to the same degree as they actually pitch to pitchers less than do starters for a number of reasons, chiefly because they are usually active in late innings when pinch hitters tend to be used in the ninth spot. ERA is also affected somewhat by the park in which a pitcher's team plays half its games as well as the tendencies of official scorers to assign errors or base hits in plays that could be either. For an extreme example, pitchers for the Colorado Rockies face a double problem. The high altitude of Denver causes fly balls to travel up to 10% farther than at sea level and reduces the ability of pitchers to throw effective breaking balls. Also, Coors Field has fences that are not long enough to compensate for the increased fly-ball distance at Denver, plus a relatively small amount of foul territory. In modern baseball, Sabermetrics utilizes several Defense independent pitching statistics in an attempt to measure a pitcher's ability regardless of factors outside his control.