Earned Run Average is baseball's oldest and most widely quoted measure of pitching performance. This guide explains exactly how ERA is calculated, what counts as an earned run, and why analysts pair ERA with WHIP, FIP, and ERA+ to get the full picture of how a pitcher is really performing.

What ERA measures and the nine-inning normalization

ERA answers a simple question: if this pitcher threw a full nine-inning game, how many earned runs would they typically allow? You take the earned runs charged to the pitcher, divide by the innings they actually pitched, and multiply by nine to scale the result to a standard game length. That multiplication is what lets you compare a reliever who threw 65 innings with a starter who threw 215 — both end up on the same per-nine-innings scale. A 3.00 ERA means roughly three earned runs per nine innings, and across modern MLB the league-average ERA hovers around 4.20.

Earned versus unearned runs

ERA only counts earned runs, which is what separates it from raw runs allowed. If a fielder boots a routine grounder and a run scores that would not have scored with clean defense, that run is 'unearned' and the official scorer removes it from the pitcher's ERA. The logic is that a pitcher should be judged on outcomes they control, not on their teammates' gloves. This is also why ERA can flatter a pitcher on a poor defensive team and why it never tells the whole story by itself — the scoring judgment of earned versus unearned introduces subjectivity that pure outcome stats avoid.

ERA's blind spots: why WHIP, FIP, and ERA+ matter

ERA is sensitive to defense, ballpark, sequencing, and luck. A pitcher can post a shiny ERA while allowing tons of base runners if those runners never come around to score, which is why WHIP — walks plus hits per inning — is a useful sanity check on traffic allowed. FIP goes further and strips out balls in play entirely, estimating an ERA from just home runs, walks, and strikeouts, the outcomes most under a pitcher's control; a FIP far below ERA suggests bad luck or weak defense, and the reverse suggests good fortune. Finally, ERA+ rescales ERA so 100 is league average and adjusts for ballpark, so a 130 ERA+ means 30 percent better than average regardless of whether the pitcher worked in a hitter's park or a pitcher's park. Reading all four together gives a far more honest verdict than ERA alone.