ERA is a measure of a pitcher's effectiveness calculated as the average number of earned runs allowed per nine innings pitched. It's one of the most widely used statistics to evaluate a pitcher's performance. Earned runs are runs that scored without the help of errors or passed balls by the defense.

The formula for ERA is: ERA = (Earned Runs × 9) ÷ Innings Pitched. This calculation normalizes a pitcher's earned run average to a standard nine-inning game, allowing for comparison between pitchers regardless of how many innings they've pitched.

Earned runs are runs that scored as a result of the pitcher's performance without the benefit of errors or passed balls. Unearned runs are runs that scored due to defensive mistakes like errors or passed balls that should have resulted in an out. Only earned runs are used in the ERA calculation.

In baseball, innings pitched are often recorded with a decimal to represent partial innings. For example, if a pitcher records 1 out in an inning, it's recorded as 0.1 innings. Two outs would be 0.2 innings. Three outs (a complete inning) would be 1.0 innings. This calculator handles these decimal values correctly in ERA calculations.

Different leagues use different standard game lengths. Major League Baseball and college baseball use 9-inning games, while high school baseball and softball typically use 7-inning games. ERA is always calculated per 9 innings for consistency, but our calculator provides context for how your ERA compares to typical performance at different levels of play.