Overview

Earned run average, commonly abbreviated ERA, is a standard baseball pitching statistic that expresses the mean number of earned runs a pitcher allows per nine innings pitched. ERA is used across levels of organized play, including baseball generally and specifically in Major League Baseball. Lower ERA values indicate better run prevention by the pitcher.

Calculation and interpretation

The basic formula is simple: divide the number of earned runs a pitcher has allowed by the total innings pitched, then multiply by nine. Innings pitched are recorded in thirds (each out equals one-third of an inning), and earned runs are those charged to the pitcher rather than attributed to defensive errors or passed balls as ruled by the official scorer.

  • Formula: ERA = (Earned Runs / Innings Pitched) × 9.
  • Example: A pitcher who allows 25 earned runs over 95 innings would have an ERA of 2.37.
  • Interpretation: ERA is normalized to a nine‑inning game to make full games and partial appearances comparable.

History and development

ERA emerged in the early 20th century as statistical record‑keeping improved and became the dominant single-number measure of a pitcher's effectiveness. It replaced cruder measures such as win–loss records or raw runs allowed, because those could be heavily influenced by how many innings a pitcher worked and by team defense.

Uses, strengths and limitations

ERA remains widely cited when describing and comparing pitchers because it is easy to compute and understand. However, it has notable limitations. Because ERA depends on which runs are ruled "earned," it is affected by the quality of a team's defense, official scoring decisions, ballpark effects, and luck on balls in play. Relievers with small inning totals can show misleadingly high or low ERAs due to limited sample size.

To address ERA's weaknesses, analysts use adjusted or alternative measures. ERA+ adjusts a pitcher's ERA for league and ballpark factors (with 100 set as league average). Fielding Independent Pitching (FIP), xFIP, and SIERA are examples of metrics designed to isolate a pitcher's performance on outcomes he can control—strikeouts, walks, and home runs—reducing the influence of defense and chance. Together, these statistics give a fuller picture of pitching value than ERA alone.