How Baseball’s Earned Run Average Works Explainer
Earned run average estimates how many earned runs a pitcher allows per nine innings. The formula is earned runs multiplied by nine, divided by innings pitched. A pitcher who allows four earned runs in six innings has a 6.00 ERA for that outing, even though a full nine innings were not thrown.
An earned run is one that would have scored without the help of fielding errors or passed balls, as reconstructed by the official scorer. Unearned does not mean the pitcher had no role in the inning. It means the scoring sequence was extended or altered by defensive mistakes that the rules exclude from ERA.
Outs are recorded in thirds of an inning, so 6.1 innings means six innings and one out, not 6.10. Relievers are charged with runners they allow to reach even if those runners score after another pitcher enters. The new pitcher can still be charged for batters faced and runners created during the relief appearance.
ERA is useful but incomplete. Defense, ballpark dimensions, league scoring levels, bullpen performance, and sequencing can influence it. It also says little about how the pitcher created outs or prevented baserunners. Analysts often combine ERA with strikeout, walk, home-run, and context-adjusted measures for a fuller picture.
Multiply earned runs by nine and divide by innings pitched. The result expresses run prevention on a nine-inning scale.
Because outs are thirds, statistical calculations treat one out as one-third of an inning and two outs as two-thirds.
The official scorer reconstructs the inning without errors or passed balls to determine which runs are earned.
A run can be unearned for ERA purposes even though the pitcher contributed to the scoring sequence.
Fielding quality, parks, league environment, inherited runners, and the order of events can change a pitcher’s ERA.
It is best read with workload and supporting statistics rather than as a complete measure of pitching skill.
Explore more "Explainers"
Discover additional explainers across politics, science, business, technology, and other fields. Each explainer breaks down a complex idea into clear, everyday language—helping you better understand how major concepts, systems, and debates shape the world around us.