ERA Calculator
Earned run average with RA9 and earned share from runs and innings.
What it does
ERA stretches any outing to a full nine innings so starters and relievers share one scale. Three earned runs across 6 innings prorate to 4.50, the classic textbook line. The tool above divides runs by innings with RA9 and earned share beside it.
The default is that exact textbook case: 3 earned of 4 total over 6 frames gives ERA 4.50 with RA9 6.00 and 75% earned. The unearned run traces to defense, which is why ERA judges arms while RA9 judges whole run prevention.
Baseball references agree on the arithmetic. The league glossary defines ERA as 9 times earned runs over innings pitched, and the stats reference works the same 3 over 6 into 4.50 step by step. Both charge runners left on base to the pitcher who allowed them aboard.
Context still rules interpretation. Parks, leagues, eras, and bullpens all tilt raw numbers, and relievers pitching fragments lean on others to strand their runners. Read ERA as the starting measure, then adjust for defense and setting.
How to use
- Type earned runs, excluding errors.
- Type total runs allowed.
- Type innings pitched in decimal outs.
- Read ERA first for the arm.
- Read RA9 for whole run prevention.
Formula
- ER
- Earned runs, no errors or passed balls.
- R
- All runs allowed.
- IP
- Innings pitched, thirds as .1 and .2.
Partial innings enter as decimal thirds: 6.1 is six and one third, 6.2 is six and two thirds. Inherited runners score against their original pitcher.
Worked example
Example: 3 earned over 6 innings
Defaults: ER 3, runs 4, IP 6.
- Divide: 3 / 6 = 0.5 per inning.
- Scale: 0.5 x 9 = 4.50 ERA.
- RA9: 4 / 6 x 9 = 6.00.
- Share: 3 / 4 = 75% earned.
- Tool shows 4.50, 6.00 RA9, 75% earned.
Reference table
| Line | ERA | Read |
|---|---|---|
| 1 ER in 9 IP | 1.00 | Elite |
| 2 ER in 9 IP | 2.00 | Strong |
| 3 ER in 6 IP | 4.50 | Average |
| 5 ER in 7 IP | 6.43 | Rough |
Limits
- Earned run judgment follows official scoring.
- Park and league context unadjusted.
- Small samples swing wildly.
- Relievers depend on strand rates.
Common mistakes
- Counting unearned runs as earned.
- Writing 6.3 innings instead of 6.1 thirds.
- Crediting relief runs to the wrong arm.
- Comparing eras across scoring eras.
- Judging relievers by ERA alone.
Try it yourself
Quick checks. Answers sit tucked under each question.
3 earned, 3 runs, 4 IP?
9 x 3 / 4 = 6.75 ERA and RA9.
2 earned, 2 runs, 9 IP?
9 x 2 / 9 = 2.00 ERA.
5 earned over 7 IP?
9 x 5 / 7 = 6.43 ERA.
Questions
ERA of 3 in 6?
4.50.
What counts as earned?
Runs without errors or passed balls.
What is RA9?
All runs per nine, earned or not.
Good ERA?
Under 3.00 territory for starters.
Why decimals like 6.1?
Baseball counts outs in thirds of innings.
Who owns inherited runs?
The pitcher who allowed the runner aboard.