By MLB.com editorial staff
ERA+ takes a player's ERA and normalizes it across the entire league. It accounts for external factors like ballparks and opponents. It then adjusts, so a score of 100 is league average, and 150 is 50 percent better than the league average.
For example, Mariano Rivera's 2.21 career ERA was 105 percent better than the MLB average during the time he pitched (including adjustments for park and league). That gives him a 205 career ERA+ (the best all-time).
League ERA, adjusted for park factors x 100 / ERA.
Why it's useful
ERA is the most universally accepted tool for measuring the performance of a pitcher. But ERA+ attempts to level the playing field for all pitchers by adjusting for the impact their home ballpark had on their performance.
This section was created by the MLB.com editorial staff for fan entertainment and education, and was not officially provided by Major League Baseball.