A pitcher's ERA measured against the league ERA of the same season, scaled so 100 is league average and higher is better: an ERA+ of 150 means the league's ERA was 1.5 times the pitcher's. As published by Baseball-Reference it is also adjusted for ballparks.
Adjusted ERA (ERA+)
Also written: ERA+
Standard statistic · MLB
Formula
ERA+ = 100 × lgERA ÷ ERA Career ERA+ weights each season's league ERA by innings pitched. (Baseball-Reference additionally applies a park factor.)
On StatVault
Computed in-house per season and per league from the published season ERA, park-unadjusted because the corpus carries no park factors; career ERA+ is innings-weighted. Shown on MLB profiles' advanced table and ranked on the MLB Advanced boards.
Shown on: MLB player profiles — advanced table