How ERA is calculated
Earned run average (ERA) is the earned runs a pitcher allows per nine innings: ERA = (earned runs ÷ innings pitched) × 9. Giving up 30 earned runs over 85 innings is (30 ÷ 85) × 9 = 3.18. Lower is better.
Two more worked examples. A starter who allows 12 earned runs across 40 innings has (12 ÷ 40) × 9 = 2.70. A rough outing of 5 earned runs in 6 innings is (5 ÷ 6) × 9 = 7.50. The multiplier is 9 because ERA is always expressed per nine-inning game, which is what lets you compare a reliever with 40 innings against a starter with 200.
Pair this with the batting average calculator when you need the fuller picture.
If the next number you need is a related one, the winning percentage calculator covers it.
How to use this calculator
Enter earned runs (runs not due to errors) and innings pitched. Enter partial innings as decimals — for example, 85⅓ innings is 85.33. The result is the ERA to two decimals.
Box scores write partial innings as .1 and .2, meaning one and two outs, so convert before entering: 85.1 becomes 85.33 and 85.2 becomes 85.67. Typing 85.1 straight in treats it as a tenth of an inning and understates the ERA slightly.
As a reference, an ERA below 3.00 is excellent, near 4.00 is roughly league average and above 5.00 is poor. ERA is also sensitive to sample size: a single bad inning early in a season can push a figure above 6.00 that settles back within a few starts, so read it alongside innings pitched rather than on its own.
The save percentage calculator handles the closely related case.
Frequently asked questions
- How do I calculate ERA?
- Divide earned runs by innings pitched and multiply by 9. 30 earned runs over 85 innings is a 3.18 ERA.
- What is a good ERA?
- In MLB, under 3.00 is excellent, around 4.00 is average, and over 5.00 is poor.
- What counts as an earned run?
- A run that scores without the help of a fielding error or passed ball.
- How do I enter partial innings?
- As decimals: one out is .33 and two outs is .67, so 85⅔ innings is 85.67.
- Do unearned runs change a pitcher's ERA?
- No. Runs that score because of a fielding error or a passed ball are unearned and are left out of the ERA formula entirely. That is why a pitcher can concede six runs in a messy inning and still see only a small rise in ERA.