ERA+ is ERA adjusted for park factor and league average, scaled so 100 equals league average and 150 means 50% better than league. ERA+ makes pitcher ERA comparable across parks and seasons.
Raw ERA varies hugely by park and era. A 3.50 ERA at Coors Field is much more impressive than a 3.50 ERA at Petco Park, where league-average pitchers post sub-4.00 ERAs by default. ERA+ corrects for that by dividing the league-average ERA by the pitcher's ERA and adjusting for the pitcher's home park.
The formula: ERA+ = 100 × (lgERA × park_factor) / pitcher_ERA. The 100 anchor means league average is always 100; higher is better. (Note the inversion vs OPS+ where higher hitter values are higher numbers; ERA+ also goes up as the pitcher gets better, even though raw ERA goes down.)
ERA+ is comparable across pitchers from different parks. A 3.20 ERA pitcher in Coors has roughly the same ERA+ as a 2.80 ERA pitcher in Oakland. Without ERA+, the betting market overrates pitchers in pitcher parks and underrates those in hitter parks.
For ranking starters across the league or comparing rotation depth between two teams, ERA+ gives true apples-to-apples.
ERA+ describes the past well. FIP and xERA predict the future better. Sharp models use FIP/xERA as primary inputs and ERA+ as a secondary descriptive check.
Park factor scales ERA+ both ways. A pitcher in Petco (park factor 0.92) gets his ERA divided by a smaller adjusted league average — making his raw 3.00 ERA look LESS impressive in ERA+ terms because the league ERA there is naturally lower. The opposite for Coors. ERA+ removes the park-dependent illusion.
ERA+ is one of the secondary pitcher inputs in the model. Primary inputs are FIP-effective, xERA, K/BB first time through the order, and lineup-vs-handedness splits. ERA+ enters as a long-horizon stability check — a pitcher with elite ERA+ over 100+ innings has demonstrated true skill, even if a 5-start sample shows variance.
Theory is one thing — using the concept on tonight's slate is the value. These hubs are the practical application: