ERA Calculator

The ERA formula

Earned run average measures how many earned runs a pitcher allows for every nine innings, the length of a standard game. The formula is:

ERA = (earned runs ÷ innings pitched) × 9

You divide earned runs by innings pitched to get the rate per inning, then multiply by 9 to scale it to a full game. Only earned runs count: a run that scores because of a fielding error or passed ball is unearned and is excluded, so ERA reflects the pitcher’s own performance rather than the defense behind them.

Worked example

Suppose a pitcher has allowed 12 earned runs across 45 innings pitched:

The ERA is 2.40, meaning the pitcher gives up about 2.4 earned runs per nine innings, a strong figure by modern standards. A starter under 3.00 is considered excellent, around 4.00 is roughly league average, and above 5.00 signals trouble.

Common pitfalls

• Counting unearned runs. Only earned runs belong in the formula. Runs that score solely because of an error do not count against ERA.
• Partial innings. Outs are thirds of an inning, not tenths. Enter 5⅓ innings as 5.333 and 5⅔ as 5.667, never as 5.3 or 5.6.
• Zero innings pitched. ERA is undefined with no innings (division by zero); a pitcher who allows runs without recording an out is conventionally shown as infinite (∞). This calculator returns 0 when innings pitched is 0.
• Tiny sample sizes. A single inning can produce a wild ERA. The stat only becomes meaningful over a reasonable number of innings.