How it works
Earned run average is how many earned runs a pitcher allows per nine innings. An earned run is one that scored without the help of an error or a passed ball, so ERA measures the runs that were the pitcher’s fault.
Innings pitched are counted in outs. Each out is one third of an inning, so box scores write 6⅓ innings as 6.1 and 6⅔ as 6.2. A calculator that reads 6.2 as six and two tenths gets the ERA wrong; this one reads it as 6⅔.
The second table shows how much one more start would move the ERA. Early in a season one bad outing swings it a lot; by September it barely moves.
The formula
ERA = earned runs × 9 ÷ innings pitched
innings pitched = outs ÷ 3 (6.1 = 6⅓, 6.2 = 6⅔)Worked example
The rulebook’s own example: 9⅓ innings pitched and 3 earned runs.
- 3 × 9 = 27.
- 27 ÷ 9⅓ = 2.89.
For a season of 180.2 innings (180⅔, or 542 outs) with 62 earned runs: 62 × 9 ÷ 180⅔ = 3.09. One more start of 6 scoreless innings would take it to 2.99.
Questions
How do I enter 6 and two-thirds innings?
Type 6.2, as a box score shows it, or 6 2/3. The digit after the point counts outs, so it can only be 0, 1 or 2.
What is the difference between earned and unearned runs?
A run is unearned when it scored because of an error or a passed ball. Unearned runs count on the scoreboard but not in ERA. The official scorer decides which runs are earned.
Who is charged when a reliever lets in inherited runners?
The pitcher who put them on base. If a starter leaves with runners on and they score, the earned runs count against the starter’s ERA, not the reliever’s.
What is a good ERA?
It depends on the league, the season and the ballpark. ERA also depends on the defense behind the pitcher, so compare pitchers in the same league and season. A lower ERA is better.
How many innings do you need to qualify for the ERA title?
In the major leagues, one inning pitched for every game on the team’s schedule: 162 innings in a 162-game season (rule 9.22). In the minor leagues it is 80% of that.
What if the pitcher has not recorded an out?
Then there is no ERA to work out yet, because it would mean dividing by zero innings. The calculator asks for at least one out.
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Sources
- Major League Baseball. Official Baseball Rules, 2026 edition. Rule 9.02(c)(1) comment (each out is one third of an inning pitched), rule 9.21 (how to compute batting average, slugging percentage, earned run average and on-base percentage, with a worked ERA example) and rule 9.22 (minimum plate appearances and innings to qualify for a title).
- MLB.com glossary: Earned Run Average (ERA). Definition, the formula 9 × earned runs ÷ innings pitched, and what affects ERA (defense, league, ballpark).
- MLB.com glossary: Innings Pitched (IP). Each out is one third of an inning; no third is credited when a batter reaches on an error.