What this calculator does
Earned run average is the standard measure of how many runs a pitcher gives up, scaled so pitchers who throw different numbers of innings can be compared fairly. It takes the earned runs a pitcher has allowed, divides by the innings they actually pitched, then multiplies by nine so the figure reads as "runs allowed per full game" regardless of how many innings were actually thrown.
The detail people get wrong is innings pitched. Baseball counts partial innings in thirds rather than tenths, so a box score showing "6.1" innings means six and one third innings, not six and a tenth, and "6.2" means six and two thirds. If you enter innings as they appear on a scoresheet, convert the digit after the decimal point first: .1 is a third of an inning, .2 is two thirds, and this calculator expects that innings figure already in decimal form (6.33 rather than 6.1) for an exact answer.
The formula
Divide earned runs allowed by innings pitched, then multiply by 9 to express the rate per standard nine-inning game. Only earned runs count, meaning runs that scored without help from a fielding error or passed ball; runs that scored because of a defensive mistake are excluded because they are not considered the pitcher's fault.
| Term | Meaning |
|---|---|
| ERA | Earned run average: (earned runs allowed ÷ innings pitched) × 9. |
| Earned runs | Runs that scored through the batting team's own play, excluding runs enabled by a fielding error or passed ball. |
| Innings pitched | The number of innings a pitcher completed, expressed in thirds (one out is 0.33, two outs is 0.67 of an inning). |
The inputs explained
| Field | What to enter |
|---|---|
| Earned runs allowed | The total earned runs the pitcher has allowed over the period being measured. |
| Innings pitched (innings) | Innings pitched in decimal form. Convert from a scoresheet reading of "x.1" or "x.2" by treating the digit after the point as thirds, not tenths. |
When to use it
Tracking a starting pitcher over a season
A season ERA accumulates earned runs and innings across every start, so recalculating it after each outing shows whether a pitcher's recent form is trending up or down.
Comparing pitchers with different workloads
A reliever who throws one inning per appearance and a starter who throws six cannot be compared on total runs allowed alone. Scaling both to a per-9-inning rate puts them on the same footing.
Projecting a full season from a partial one
A pitcher's ERA after a handful of starts is a small sample and can move sharply with one bad outing, so it is worth checking against a full season of innings before drawing conclusions.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How ERA changes with earned runs allowed, over 200 innings pitched
A pitcher who has thrown 200 innings, at a range of earned runs allowed.
How ERA changes with innings pitched, at 60 earned runs allowed
The same 60 earned runs, spread across a range of innings pitched.
Questions
Why does innings pitched use .1 and .2 instead of decimals?
Because an inning has three outs, a partial inning is naturally a third or two thirds, not a tenth. Scoresheets write "6.1" to mean six innings plus one out, which this calculator treats as 6.33 innings, not 6.1.
Why are only earned runs counted, not all runs?
ERA is meant to measure the pitcher's own performance, so runs that scored because of a fielding error or passed ball are excluded. The total-runs figure including those is usually tracked separately as RA9.
What counts as a good ERA?
It varies by era, league and ballpark, but as a rough guide a starting pitcher's ERA under 3.00 across a full season is considered excellent, under 4.00 is solid, and above 5.00 is generally considered poor.
Does ERA punish a reliever the same way as a starter?
The formula is identical for both, but a reliever who pitches only a handful of innings per appearance can see their ERA swing far more sharply from a single bad outing than a starter's can, since the denominator is so much smaller.
For the cricket equivalent of comparing team scoring rates, see the net run rate calculator.