What is ERA in Baseball

ERA, or earned run average, is one of the most commonly accepted statistical tools for understanding a pitcher’s effectiveness in baseball. It shows how many earned runs a pitcher allows per nine innings pitched, which is the traditional length of a game. To find ERA, the formula is straightforward: take the number of earned runs a pitcher has allowed, divide it by innings pitched, and then multiply by nine. This calculation excludes any runs scored with the aid of a passed ball or defensive errors, focusing only on earned runs that are fully the pitcher’s responsibility.

In Major League Baseball, ERA remains a key way to evaluate pitchers, but it has its flaws. For instance, while defensive mistakes like errors are accounted for, great defensive plays are not. This puts a pitcher with average defense at a disadvantage compared to one with a strong defense behind him. Additionally, it’s tough to evaluate ERA across different leagues and ballparks, as National League pitchers might see lower ERAs without a designated hitter. Certain stadiums can also affect ERA by being more favorable to run scoring, impacting the numbers for pitchers who play there often.

ERA is particularly useful for measuring a starting pitcher’s success over the course of a game, as it reveals how many runs he typically allows due to his own skill, not outside influences. However, it’s not as effective for relief pitchers since they usually only pitch a few innings or even fractions of an inning. This means their ERA might be shaped by other relievers who inherit their base runners. Because relief pitchers often use all their energy on a few outs, their ERAs tend to be lower than those of starting pitchers, who must conserve their energy to last through more innings.
While ERA measures a pitcher’s effectiveness by calculating the average runs allowed, RBI (Runs Batted In) provides insights into a batter’s ability to drive in runs. If you’re interested in understanding how RBI complements ERA in evaluating player performance, check out our article on RBI in baseball

Origins

Henry Chadwick, a statistician and writer, is often credited with devising the statistic that we know as ERA (earned run average). Back in the mid-to-late 19th century, Chadwick believed that a win-loss record alone was not enough for determining how good a pitcher truly was. His thinking laid the foundation for using ERA as a clearer measure of a pitcher’s effectiveness. This concept caught on as relief pitching became more prevalent in the 20th century, shifting the focus from solely win-loss records to a broader assessment of performance.

In the early 1900s, pitchers were still routinely expected to pitch complete games, and their win-loss record was considered a sufficient criterion for determining effectiveness. However, as more relief specialists like James Otis Crandall and Charley Hall emerged, it became clear that simply tabulating wins and losses wasn’t enough to capture a pitcher’s responsibility in team contributions. This led to the idea that earned-run responsibility should be split across pitchers in games that included multiple contributors, especially when opposing batters reached base or scored due to the initial pitcher’s work. By 1912, the National League officially tabulated ERA statistics (originally known as “Heydler’s statistic” after John Heydler, then NL secretary), which quickly spread to the American League and became an accepted standard across baseball.

Today, baseball encyclopedias often display ERAs even for earlier years, though these numbers were computed retroactively. While the Negro leagues had fewer recorded statistics, pitchers there are often rated by RA (total runs allowed) instead of ERA, as Negro league games didn’t always distinguish between earned and unearned runs.

As with batting average

The definition of a good ERA in baseball can change depending on the year and conditions of the game. For example, during the dead-ball era in the 1900s and 1910s, an ERA below 2.00 was considered good because earned runs were rare, and pitchers had more control over hitters. However, by the late 1920s and 1930s, when conditions favored hitters, only the highest caliber pitchers like Dazzy Vance or Lefty Grove could keep an ERA below 3.00. By the 1960s, an ERA under 2.00 returned briefly due to new influences like ballparks with different dimensions, and today, starting with the 2019 season, an ERA under 4.00 is again viewed as good for most pitchers.

Some of the best single-season ERA records belong to legends like Dutch Leonard, who, in 1914, posted an earned run average of 0.96 over 224.2 innings with a win–loss record of 19–5. Bob Gibson holds the record for the lowest single-season ERA with 1.12 in 1968 for pitchers with at least 300 innings. The lowest career ERA in history, 1.82, was set by Ed Walsh, who played from 1904 through 1917. These impressive numbers show how the best pitchers have achieved remarkable control over earned runs and innings across different eras.

Infinite and undefined

In some rare cases, a pitcher may end up with an infinite ERA. This happens when players give up earned runs without retiring a batter, resulting in a divide by zero scenario because no outs are recorded. For instance, Bill Ford with the Boston Bees in 1936 and Fred Bruckbauer of the Minnesota Twins in 1961 each had a single appearance in MLB and recorded an infinite ERA for their limited pitching careers. Similarly, outfielder Vic Davalillo made pitching appearances for the St. Louis Cardinals in 1969 but failed to get an out after facing several batters, allowing a run and leaving him with an infinite ERA as a major-league pitcher.

An undefined ERA can also show up early in a baseball season if a pitcher has yet to appear in games. Sometimes this undefined value is incorrectly displayed as zero or given the lowest-ranking spot. This misrepresentation can cause confusion, especially when players or fans check the ERA standings at the beginning of a season.

Other external factors – Starters and relievers

Sometimes, it can be misleading to judge relief pitchers based on ERA alone because they are not always charged for every run they allow. If a relief pitcher enters a game with bases loaded and the team leading by 1 run, he might face just one batter who hits a single that scores 2 runs. Even though he may retire the next batter, his ERA could still show 0.00 for that game if he only pitches briefly and doesn’t record any outs for those runs. This situation would still count as a blown save if those runs tied the game or led to a lead change.

For starting pitchers, the rules are slightly different since they begin pitching with no runners on base. Relief pitchers, on the other hand, know they will likely be pitching for a short time and can put in maximum energy to get a few batters out quickly. In contrast, starters have to conserve energy to last 7 innings or more in a game. This freedom allows relievers to focus on a few outs, which generally helps keep their ERAs lower than starting pitchers. So, using ERA to objectively judge both types of pitchers might not capture their roles accurately.

Designated hitter rule

The advent of the designated hitter rule in the American League in 1973 changed the pitching environment significantly. With this rule, AL pitchers didn’t have to focus on batting, unlike those in the National League who still had to bat and often got an easy out when facing the opposition’s pitcher. This difference put American League pitchers at a slight disadvantage when it came to maintaining low ERAs, as they faced one more skilled batter. By 1997, when teams began playing interleague games during the regular season, the DH rule applied only in American League parks. However, this was rendered moot in 2022 when the National League also adopted the DH permanently.

This difference between leagues didn’t affect relievers to the same degree as starters, since NL relievers rarely faced pitchers directly. Instead, they often encountered pinch hitters in the pitcher’s batting spot, especially due to the double switch tactic commonly used in the later innings. So, while the designated hitter rule had a noticeable impact on ERAs, it shaped starters’ and relievers’ roles in somewhat unique ways across both leagues.

Location

A pitcher’s ERA can be strongly affected by the ballpark where their team plays its games. For instance, pitchers with the Colorado Rockies face unique problems due to Denver’s high altitude of 5,280 ft (1,610 m), combined with its semi-arid climate. At this altitude, fly balls tend to travel about 10% farther than at sea level, making it harder to control ERAs. The low humidity also affects breaking balls by reducing air resistance and creating difficulty gripping dry baseballs. Since 2002, the Rockies have used humidors to store baseballs before games to counteract these conditions, but the challenges remain significant.

These conditions make it particularly difficult for Rockies’ pitchers to keep earned runs low, even if opposing pitchers experience similar issues. However, hometown hurlers may develop techniques to adjust, gaining a slight advantage from being more physically acclimated to the altitude. This familiarity doesn’t eliminate the issue, though, as the conditions in Denver still tend to inflate Rockies’ ERAs compared to the rest of the league. Official scorers and errors assigned to certain plays can also add complexity to how ERA is calculated in different ballparks

Sabermetric treatment of ERA

In modern baseball, sabermetrics has introduced Defense-Independent Pitching Statistics (DIPS) to provide a more accurate way to measure a pitcher’s ability by focusing only on what they can control. Since ERA depends on factors beyond the pitcher’s control, analysts now use other performance indicators like strikeout and walk rates to improve forecasting of future ERAs. For example, Nate Silver’s PECOTA system provides forecasts of ERAs using more reliable predictors, as Silver also developed a “quick” earned run average called QuikERA (QERA), which calculates ERA based on peripheral statistics such as strikeouts, walks, and groundball percentage.

Another statistic derived from ERA is Adjusted ERA (ERA+), which adjusts a pitcher’s ERA to a scale where 100 is average across the league, considering each ballpark’s dimensions and park effects. Additionally, a statistic called R/9 (RA9) measures a pitcher’s effectiveness at preventing runs by looking at total runs allowed per nine innings, not just earned runs. By using these advanced metrics, sabermetrics has expanded how we view pitching statistics and given fans and teams a more precise way to judge pitchers.

Career leaders in the live-ball era (post-1920, Starting Pitchers Only)

In post-1920 baseball, several rules changes have impacted ERA. With the abolition of the spitball and the frequent replacement of soiled or scuffed baseballs, pitchers had to adjust their approach. The increased importance of the home run—inspired largely by Babe Ruth—and the American League’s adoption of the designated hitter rule also made it harder for pitchers to keep their ERAs low. Over these decades, ERAs became noticeably higher than in earlier years of the sport.

When looking at a list of the lowest ERAs among pitchers who spent their entire careers after 1920, there are some standout performances. Only starting pitchers with a minimum of 1,500 innings pitched are considered in these rankings. These pitchers managed to hold impressive ERA records despite the evolving game dynamics, showing consistent skill and adaptation in response to the new challenges and influences in the game.

F&Q

What is a good ERA in baseball?

In modern baseball, an ERA under 2.00 is considered exceptional and is rare, only achieved by the very best pitchers in the league. If a pitcher allows one or more runs without retiring a batter in a single appearance, it can significantly impact his ERA. Generally, an ERA between 2.00 and 3.00 is still considered excellent and reflects a pitcher’s high skill level.

How does ERA in baseball work?

ERA is a commonly accepted statistical tool for evaluating pitchers. The formula for finding ERA is straightforward: take 9, multiply it by the earned runs a pitcher has allowed, and then divide by innings pitched. If a pitcher exits a game with runners on base, any earned runs scored by those runners still count against his ERA.

What is the highest ERA in baseball?

Since 1920, the best ERA in baseball history for a Hall of Fame closer belongs to Mariano Rivera, with an impressive ERA of 2.21 over 1,000 innings pitched.

How do you calculate ERA+?

League adjusted ERA or ERA+ is calculated as 100 times (lgERA/ERA) on Baseball Reference. This stat adjusts for a player’s ballpark and takes into account a park factor of one, meaning no advantage or disadvantage from pitching in that stadium.