The ERA formula
ERA is earned runs multiplied by nine, divided by innings pitched. The nine scales a pitcher's performance to the length of a regulation game.
How partial innings are written
Baseball records outs, not decimal tenths. Six and one-third innings is written 6.1, and six and two-thirds is written 6.2. A value such as 6.3 is invalid because the third out completes the seventh inning.
Earned and unearned runs
ERA uses earned runs. An official scorer determines whether a run would probably have scored without an error or passed ball. A run can also be charged to a pitcher after that pitcher leaves if a responsible baserunner later scores.
What counts as a good ERA?
Lower is generally better, but there is no timeless universal cutoff. League scoring environment, era, ballpark, defence, starting versus relief role and sample size all change interpretation.
ERA does not measure everything
ERA depends partly on fielding and sequencing. Other statistics can add context, including WHIP, strikeout and walk rates, FIP and ERA+. A small number of innings can also produce an unstable result.