Earned runs per nine innings
ERA scales a pitcher\'s earned runs up to a full nine-inning game so any two pitchers compare fairly. 72 earned runs over 180 innings is (72 × 9) ÷ 180 = 3.60. Only earned runs count: runs that score because of an error or passed ball are unearned and left out.
Fractional innings: .1 and .2 mean thirds
- The decimal is outs, not tenths. Innings are logged as whole innings plus outs recorded in the next one. 180.1 means 180 and one out, or 180⅓ innings; 180.2 means 180 and two outs, 180⅔.
- Convert before dividing. Treat .1 as +0.333 and .2 as +0.667. Using 180.1 as if it were 180.10 skews the result.
- A perfect outing is 0.00. Give up no earned runs and ERA is zero; it can never go negative.
How to read an ERA
| ERA | Level |
|---|---|
| Under 2.00 | Exceptional, rare even among aces |
| 2.00 – 2.99 | Excellent, Cy Young territory |
| 3.00 – 3.99 | Above average, All-Star level |
| 4.00 – 4.99 | Around league average |
| 5.00 and up | Below average |
Context matters: current MLB league average sits near 4.20–4.50, so a 3.00 is stronger in a high-offense season than in a pitcher-friendly one. ERA+ rescales for exactly this, putting 100 at league average.
Common questions
Why is ERA multiplied by 9?
A standard game is nine innings, so multiplying by 9 states the number as earned runs allowed per complete game. That lets you compare a pitcher who threw 50 innings with one who threw 200 on the same scale.
What is the difference between earned runs and ERA?
Earned runs are a raw count. ERA is the rate: earned runs per nine innings. 50 earned runs in 100 innings gives the same 4.50 ERA as 100 earned runs in 200 innings.
What counts as a good ERA?
Under 3.00 is excellent, the 3s are above average, and around 4.20 to 4.50 is roughly the current MLB league average. Relievers are held to a stricter standard than starters, and a low-offense era makes the same number look less impressive.
