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In baseball, an earned run is any run for which the pitcher is held accountable (i.e., the run scored as a result of normal pitching, and not due to a fielding error or a passed ball). All others are unearned runs. An error made by the pitcher in fielding his position is counted the same as an error by any other player.

Earned runs are specially denoted because of their use in calculating a pitcher's earned run average – the number of earned runs allowed per 9 innings pitched.

To determine whether a run is earned, the official scorer must reconstruct the inning as it would have occurred without the errors (for purposes of this rule, the "errors" also include passed balls). The benefit of the doubt is always given to the pitcher in determining which bases would have been reached by errorless play.

If no errors occur during the inning, all runs are automatically earned. In a few cases, an error can be rendered harmless while the inning is still going on. For example, a runner on first base advances to second on a passed ball. The next batter walks. Since the runner would now have been at second anyway, the passed ball no longer has any impact on the earned/unearned calculation.

A run is counted as unearned when:

• A batter reaches base on an error, and later scores a run in that inning.
• A batter or runner advances one or more bases on an error and scores on a play that would otherwise not have provided the opportunity to score.
• A baserunner scores after the third out would have been made.

In the first two cases above, "on an error" includes situations where the batter makes a clean hit, but should have been out earlier in his at bat on a foul fly ball which was dropped by a fielder for an error.

While the inning is still being played, this last scenario can cause a temporary situation where a run has already scored, but its earned/unearned status is not yet certain. For example, with two outs, a runner on third base scores on a passed ball. For the time being, the run is unearned since the runner should still be at third. If the batter strikes out to end the inning, it will stay that way. If the batter gets a base hit, which would have scored the runner anyway, the run now becomes earned.

When pitchers are changed in the middle of an inning, and one or more errors have already occurred, it is possible to have a run charged as earned against a specific pitcher, but unearned to the team. The simplest example is when the defensive team records two outs and makes an error on a play that would have been the third out. A new pitcher comes into the game, and the next batter hits a home run. The runner who reached on the error comes around to score, and his run is unearned to both the prior pitcher and the team. However, the run scored by the batter is counted as earned against the relief pitcher, but unearned to the team (since there should have already been three outs). Had the team not switched pitchers, neither run would be counted as an earned run because that pitcher should have already been out of that inning.

A pitcher is only charged with the number of runners that reached base while he was pitching. When a pitching change occurs, the new pitcher is said to "inherit" any runners that are on base at the time, and if they later score, those runs are charged (earned or unearned) to the prior pitcher. Most box scores now list inherited runners, and the number that scored, as a statistic for the relief pitcher.

In the early history of major league baseball, the difference between the number of earned runs given up by a pitcher and the total number of runs given up was much more significant than today. For instance, Jim Devlin in 1876 pitched 66 complete games (662 innings pitched) with a 1.56 ERA but managed to record only five shutouts. The seeming discrepancy comes from the difference in the number of allowed runs (309) versus earned runs (108).

• Major League Baseball Rule 10.16 - Determining Earned Runs

• Baseball statistics
• Earned run average
• Run average