What is Earned Run Average (ERA) in Baseball
Earned Run Average (ERA) is a baseball pitching statistic that measures the average number of earned runs a pitcher allows per nine innings pitched, and is the primary measure of a pitcher's effectiveness.
Earned Run Average, universally known as ERA, is the primary statistic used to evaluate a pitcher's performance. It measures how many earned runs a pitcher gives up per nine innings pitched. The lower the ERA, the more effective the pitcher. ERA is one of the fundamental statistics in baseball and has been tracked since the 19th century.
The Formula
$$\text{ERA} = \frac{\text{Earned Runs} \times 9}{\text{Innings Pitched}}$$
Earned runs are runs that scored without the benefit of a fielding error or passed ball — runs the pitcher is personally responsible for allowing.
Example Calculation
A pitcher throws 200 innings and allows 60 earned runs: $$\text{ERA} = \frac{60 \times 9}{200} = \frac{540}{200} = 2.70$$
ERA Scale — What Is a Good ERA?
| ERA | Assessment |
|---|---|
| Under 2.00 | Elite — historically rare and dominant |
| 2.00–2.99 | Excellent — typically ace-level starter |
| 3.00–3.74 | Very good — solid frontline pitcher |
| 3.75–4.49 | Average to above average |
| 4.50–5.00 | Below average — back of rotation starter |
| 5.00+ | Poor — at risk of losing roster spot |
League-average ERA fluctuates by era: during the Steroid Era (1990s–early 2000s), league ERA climbed above 4.50; in the deadball era (early 1900s) it was under 3.00.
ERA vs. FIP (Fielding Independent Pitching)
ERA has a significant limitation: it can be affected by the quality of the defense behind the pitcher. A pitcher with poor fielders behind him may have an inflated ERA through no fault of his own.
FIP (Fielding Independent Pitching) estimates what a pitcher's ERA should be based only on outcomes the pitcher controls: strikeouts, walks, hit batters, and home runs — eliminating defensive influence.
ERA+ (ERA Plus)
ERA+ is an adjusted version that accounts for the pitcher's home ballpark and the league average ERA, allowing fair comparison across eras and parks:
- ERA+ of 100 = exactly league average
- ERA+ of 130 = 30% better than league average
- ERA+ of 70 = 30% worse than league average
Historical Context
The all-time MLB record for lowest career ERA belongs to Ed Walsh (1.82), though modern ERA leaders like Clayton Kershaw and Jacob deGrom have produced historic seasons in the modern era.