How this instrument works
Earned run average (ERA) is earned runs allowed divided by innings pitched, multiplied by nine. The multiplication by nine is what makes the number useful for comparison: instead of reporting 'runs per inning pitched,' which would be a small, awkward decimal, ERA reports runs per nine innings — a full standard game — so a starter who pitches seven innings a start and a reliever who pitches one can both be read on the same familiar scale.
'Earned' is the key word doing the work. A run is earned if it scored through the batting side's own offense — hits, walks, sacrifices — without help from a fielding error or passed ball along the way. Runs that score because of a defensive mistake are unearned and excluded from ERA entirely, on the reasoning that a pitcher shouldn't be statistically penalized for a teammate's dropped ball. This is also why ERA needs a judgment call from the official scorer on every run, unlike a purely mechanical stat like batting average.
In modern MLB, an ERA in the high 3s to low 4s is roughly league average for a starting pitcher, a mark below 3.00 is considered excellent, and anything below 2.00 across a full season is an outstanding, headline-making year. Because ERA is a rate stat scaled to nine innings, it stays meaningful whether a pitcher has thrown 40 innings or 200 — which is exactly the comparison batting average, an unscaled ratio, can't offer across roles.
- Enter Earned runs allowed — runs that scored without the help of a fielding error or passed ball.
- Enter Innings pitched — as a plain decimal number, not baseball's box-score notation (see the FAQ below on partial innings).
- Read Earned run average (ERA) beneath the inputs, shown to two decimal places.
- Innings pitched must be greater than zero — ERA is undefined for a pitcher who hasn't recorded an out.
Worked example — 40 earned runs over 120 innings
Enter 40 into Earned runs allowed and 120 into Innings pitched. ERA = (40 / 120) × 9 = 3.00 — a strong full-season ERA, comfortably better than league average for a starting pitcher.
Scale the innings up while holding the same rate and the ERA doesn't move: 60 earned runs over 180 innings is the identical ratio, still 3.00. That's the entire point of the ×9 scaling — it lets a shorter, injury-shortened season and a full 180-plus-inning workload be compared on equal footing, provided the underlying rate of runs allowed is the same.
Questions
What's the difference between an earned run and an unearned run?
An earned run scores through the batting side's own offense — hits, walks, hit-by-pitches, sacrifices — with no fielding error or passed ball contributing along the way. If a defensive miscue extends the inning or lets a runner advance who otherwise wouldn't have, any run that scores as a result is ruled unearned and excluded from ERA, since the pitcher isn't considered responsible for a teammate's mistake.
How do I enter partial innings pitched — what does '.1' or '.2' mean?
In baseball box scores, innings pitched is written with a fractional-looking notation where .1 means one out recorded (a third of an inning) and .2 means two outs recorded (two-thirds of an inning) — it is not a decimal. This instrument takes Innings pitched as a plain decimal number, so a pitcher credited with 120 innings and 1 out in box-score notation (120.1) should be entered here as 120.33, not 120.1.
What is considered a good ERA?
In modern MLB, an ERA in the high 3s to low 4s is roughly league-average for a starting pitcher, an ERA below 3.00 is considered excellent, and a full season below 2.00 is an outstanding, headline year — the 2.25 ERA in this instrument's second reference scenario (25 earned runs over 100 innings) sits comfortably in that elite range.
Why is ERA scaled to nine innings instead of just reporting runs per inning?
Because runs allowed per single inning would be a small, hard-to-compare decimal, and because nine innings is the length of a standard game — scaling to that length lets a two-inning relief appearance and a complete-game start be read on the same familiar number line, which a raw per-inning rate wouldn't naturally provide.
Does ERA account for the quality of a pitcher's defense?
Only partially, through the earned/unearned distinction — runs directly caused by a fielding error are excluded. But ERA still doesn't correct for a defense that gets to fewer balls in the first place (rather than mishandling the ones it reaches), which is one reason advanced pitching metrics like FIP were developed to isolate a pitcher's performance further from their fielders'.