Enter a pitcher's ERA, the league average ERA, and the park factor to calculate their ERA+ — an adjusted metric that accounts for ballpark effects and league context. A score of 100 is exactly league average; anything above 100 means the pitcher outperformed the league, anything below means the opposite. Great for comparing pitchers across different eras and run environments.
ERA+
--
Performance Rating
--
vs. League Average
--
Park-Adjusted ERA
--
ERA+ (Adjusted Earned Run Average) is a pitching statistic that normalizes a pitcher's ERA relative to the league average and the park factor of their home ballpark. A score of 100 is exactly league average. A score of 130 means the pitcher was 30% better than the average pitcher that season, after accounting for ballpark effects.
ERA+ is calculated as: ERA+ = 100 × (2 − (ERA ÷ League ERA) × (1 ÷ Park Factor)). This formula adjusts a pitcher's raw ERA for both the run-scoring environment of their home ballpark and the overall offensive level of their league.
A park factor of 1.00 means the ballpark is completely neutral — it neither favors hitters nor pitchers. Values above 1.00 (like 1.10) indicate a hitter-friendly park, while values below 1.00 (like 0.92) indicate a pitcher-friendly park. The park factor adjusts ERA+ so pitchers in hitter-friendly parks aren't unfairly penalized.
100 is the baseline (league average). An ERA+ of 120–140 is considered very good; 150+ is elite. All-time greats like Pedro Martínez and Walter Johnson posted career ERA+ values above 150. Anything below 100 means the pitcher performed worse than the league average after park adjustments.
Raw ERA doesn't account for the run-scoring environment or the ballpark a pitcher plays in. A 3.80 ERA at Coors Field (a notoriously hitter-friendly park) is far more impressive than a 3.80 ERA at Petco Park. ERA+ levels the playing field, making it possible to compare pitchers across different teams, stadiums, and even different eras of baseball.
ERA measures how many earned runs a pitcher allows per nine innings. ERA+ adjusts that number for league context and park effects, then scales it so 100 is always the average. ERA is an absolute measure; ERA+ is a relative one, making cross-era and cross-park comparisons meaningful.
Yes, ERA+ applies to any pitcher with recorded ERA data, including relievers. However, small sample sizes make ERA+ less reliable for pitchers who throw very few innings. Relievers with fewer than 20–30 innings pitched may show extreme ERA+ values that don't reflect true skill.
Baseball Reference (baseball-reference.com) is the most reliable source for both metrics. League average ERA by year is listed in their season-level stats pages. Park factors for every MLB ballpark are published annually on their team and park pages.