Guide
How to calculate ERA
A clear walkthrough of the ERA formula, baseball innings notation, and worked examples — the same math used by the calculator.
How it works
What is ERA?
Earned run average (ERA) is the average number of earned runs a pitcher allows per nine innings. It answers a simple question: if this pitcher threw a full game, how many earned runs would you expect them to give up at their current rate?
An earned run is any run that scores without help from a fielding error or a passed ball. The official scorer decides which runs are earned; unearned runs still count on the scoreboard but are left out of ERA, so a pitcher isn't charged for their defense's mistakes.
ERA formula
ERA = Earned Runs × 9 ÷ Innings Pitched
Because innings are recorded in thirds, the most reliable way to apply the formula is with outs. Nine innings contain 27 outs, so the equivalent formula is ERA = earned runs × 27 ÷ outs. This calculator always works in outs, then rounds the result to two decimal places.
For leagues that play shorter games, replace 9 with the game length — for example earned runs × 7 ÷ innings pitched for seven-inning games.
How to calculate ERA
- Total the earned runs charged to the pitcher. Leave out unearned runs.
- Convert innings pitched to outs. Multiply full innings by 3 and add the digit after the dot: 36.0 is 108 outs, 6.2 is 20 outs.
- Multiply earned runs by 27 (or by 21 for seven-inning games).
- Divide by outs and round to two decimals.
Example: 12 earned runs in 36 innings → 12 × 27 ÷ 108 = 3.00. That's identical to 12 × 9 ÷ 36, because 36 innings is exactly 108 outs.
Baseball innings explained
Innings pitched use baseball notation, not ordinary decimals. The number after the dot counts outs recorded in an unfinished inning: .0 means a complete inning, .1 means one out and .2 means two outs. A third out completes the inning, so 5.2 is followed by 6.0 — there is no 5.3.
| Written as | Means | Outs | Not |
|---|---|---|---|
| 5.0 | 5 innings | 15 | — |
| 5.1 | 5⅓ innings | 16 | 5.1 decimal innings |
| 5.2 | 5⅔ innings | 17 | 5.2 decimal innings |
| 6.0 | 6 innings | 18 | — |
Treating notation as decimals quietly skews the result. With 1 earned run in 6.1 innings, the correct ERA is 1.42; plugging in 6.1 as a decimal gives 1.48. When you add innings together, add outs: 5.2 + 1.1 is 17 + 4 = 21 outs, or 7.0 innings.
ERA examples
Each row below is calculated with the same code that powers the calculator.
| Line | ER | IP | Calculation | ERA |
|---|---|---|---|---|
| Quality start | 3 | 7.0 | 3 × 27 ÷ 21 outs | 3.86 |
| Short start | 2 | 5.2 | 2 × 27 ÷ 17 outs | 3.18 |
| One out into the 7th | 1 | 6.1 | 1 × 27 ÷ 19 outs | 1.42 |
| Complete-game shutout | 0 | 9.0 | 0 × 27 ÷ 27 outs | 0.00 |
| Full season | 45 | 150.1 | 45 × 27 ÷ 451 outs | 2.69 |
| High school season (7-inning) | 10 | 35.0 | 10 × 21 ÷ 105 outs | 2.00 |
The same high school line on a nine-inning scale would be 2.57, which is why it matters to compare ERAs calculated the same way.
ERA vs WHIP
WHIP (walks plus hits per inning pitched) is (walks + hits) ÷ innings pitched. Where ERA counts runs that actually scored, WHIP counts how many batters reached base by a hit or walk, whether or not they came around to score.
The two often move together, but not always: a pitcher who allows plenty of baserunners and strands them can post a low ERA with a high WHIP. Looking at both gives a fuller picture than either alone.
ERA vs FIP
FIP (fielding independent pitching) uses only strikeouts, walks, hit batters and home runs — outcomes that don't involve the fielders — and adds a league constant so the result reads on the same scale as ERA.
ERA describes what happened, including the effects of defense and sequencing. FIP estimates what a pitcher might have allowed with average defense and luck on balls in play. Neither is “the right one”; a large gap between them is a prompt to look closer. Back to the ERA calculator.