Understanding ERA in baseball
What earned run average measures, how earned and unearned runs differ, why innings use thirds, and where ERA falls short as a pitching stat.
What ERA measures
Earned run average expresses how many earned runs a pitcher gives up per full game, normally nine innings. Because it is a rate rather than a raw total, it lets you compare a reliever who throws 60 innings with a starter who throws 200. The formula multiplies earned runs by the game length and divides by innings pitched, so a starter allowing 30 earned runs across 180 innings posts a 1.50 ERA. The lower the number, the fewer runs the pitcher is responsible for over a standard game.
Earned versus unearned runs
Only earned runs count toward ERA, and the distinction matters. A run is unearned when it would not have scored without a fielding error or a passed ball, so a clean inning ruined by a dropped fly ball may charge the pitcher with nothing. Official scorers reconstruct each inning as if the defense had played errorlessly to decide which runs are earned. This is why two pitchers with identical runs allowed can carry very different ERAs.
Why innings use thirds
An inning ends after three outs, so partial innings are counted in thirds of an inning, one third per out recorded. Box scores write these as .1 and .2, meaning one and two outs, which trips up anyone who reads them as tenths. To get an accurate ERA you convert those thirds to decimals, so 6.1 becomes 6.333 and 6.2 becomes 6.667 before dividing. This tool expects the decimal form and reminds you of the conversion beneath the result.
The limits of ERA
ERA is easy to read but blind to a lot. It credits or blames a pitcher for the defense behind them, ignores how runners left on base are handled by relievers, and swings with the luck of balls in play. Modern analysis leans on fielding independent stats like FIP, which strip out defense, alongside WHIP and strikeout and walk rates. Treat ERA as a useful headline number rather than the final word on how well someone pitched.