ERA (Earned Run Average) has been one of the most common baseball stats for over a century. Like batting average, which we covered last week, its origin dates back to Henry Chadwick in the 1800s, who designed the stat with the purpose of illustrating the amount of runs a pitcher will give up per 9 innings that they pitch.
The formula:
9 x EARNED RUNS / INNINGS PITCHED = ERA
Notice we are using earned runs and not simply runs. Unearned runs do not count towards a pitcher’s ERA.
Since ERA deals with runs allowed, a lower number is obviously better.