Earned Run Average (ERA) Calculator
Earned Run Average (ERA) Calculator
Earned runs times nine, divided by innings pitched — worked from the innings and outs, not from the pitching line read as a decimal. A line reading 5.2 is five innings and two outs, and dividing by 5.2 makes the ERA about 9% too high at that length.
ERA from earned runs, innings and outs
Earned runs per game3 earned runs in 9 complete innings and 1 further out, nine-inning game — the example worked in the Rule 9.21(e) Comment
Formula, the constant, and the notation it has to survive
- Rule 9.21(e)
- “multiply total earned runs by 9 and divide by total innings pitched”, fractions included; the rule’s own Comment works 3 earned runs in 9 1/3 innings as 2.89. The 9 is the innings in a complete game, not a constant of the sport, so it is a field above
- outs ÷ 3, not the figure after the point
- a line reading 5.2 is five innings and two outs — 5.667 innings. Dividing by 5.2 understates the denominator by 7/30 of an inning per out, making the ERA 8.97% too high at that length. And 5.3 is not a pitching line at all, because the third out finishes the inning: a calculator that accepts it is computing something that cannot have happened
- “earned” is a judgement
- Rule 9.16 makes the official scorer replay the inning as though the defence had played without errors or passed balls and decide which runs would still have scored. Two scorers can file different numbers for one inning
Worked example
3 earned runs in 9 complete innings and 1 further out, nine-inning game — the example worked in the Rule 9.21(e) Comment
Innings pitched = 9 + 1 ÷ 3 = 9.333 innings, 28 outs
ERA = 3 × 9 ÷ 9.333 = 2.89 — the figure the Rule 9.21(e) Comment itself gives, reproduced as a check on the formula
Read the line as a decimal instead and you get 3 × 9 ÷ 9.1 = 2.97, which is 2.56% too high
Double both sides and nothing moves: 6 earned runs in 18 innings and 2 outs is 2.89 again. Halve the innings and it doubles: 3 in 4 innings and 2 outs is 5.79. Switch the full game to seven innings and the same pitching reads 3 × 7 ÷ 9.333 = 2.25 — the pitcher did not change, the unit did
At 0 innings and 0 outs the page returns no figure at all, because earned runs per no innings does not exist and 0 ÷ 0 is not zero
What the figure after the point actually means
| Line reads | Innings and outs | True innings | Outs |
|---|---|---|---|
| 5.0 | 5 innings | 5.000 | 15 |
| 5.1 | 5 innings 1 out | 5.333 | 16 |
| 5.2 | 5 innings 2 outs | 5.667 | 17 |
| 228.2 | 228 innings 2 outs | 228.667 | 686 |
The size of the notation error is NOT one percentage
| Innings pitched | True innings | Divided as a decimal | ERA overstated by |
|---|---|---|---|
| 0 innings 2 outs | 0.667 | 0.2 | 233.33% |
| 1 inning 2 outs | 1.667 | 1.2 | 38.89% |
| 5 innings 2 outs | 5.667 | 5.2 | 8.97% |
| 9 innings 2 outs | 9.667 | 9.2 | 5.07% |
| 9 innings 1 out | 9.333 | 9.1 | 2.56% |
| 20 innings 2 outs | 20.667 | 20.2 | 2.31% |
| 200 innings 2 outs | 200.667 | 200.2 | 0.23% |
| 180 innings 1 out | 180.333 | 180.1 | 0.13% |
Why an ERA needs two fields, and what “earned” is hiding
Rule 9.21 of Major League Baseball’s Official Baseball Rules, 2024 Edition, is the one place these percentages are defined by the body that runs the competition rather than by statistical custom. It sets out winning percentage at 9.21(a), batting average at (b), slugging percentage at (c), fielding average at (d), earned-run average at (e) and on-base percentage at (f). Earned-run average is subsection (e): earned runs multiplied by nine and divided by innings pitched, fractions included. MLB’s glossary says the same in the same words — “The formula for finding ERA is: 9 x earned runs / innings pitched” — and neither source explains the nine.
It needs explaining, because the nine is not a constant of the game: it is the innings in a complete game where you happen to be looking. NFHS Rule 4-2-1 makes a US high school game seven, “A regulation interscholastic game consists of seven innings (turns at bat)”, and MLB itself played seven-inning doubleheaders from the shortened 2020 season through 2021, which did not return for 2022. One further dated change is worth knowing: the automatic runner placed on second to start each extra inning was “instituted in 2020 in an effort to preserve player health and safety during the pandemic-shortened season”, was made permanent by MLB’s competition committee on 13 February 2023 and “applies only to regular-season games”. Whether runs that runner scores are earned could not be established from the rulebook here, for the reason the references give, so nothing is claimed about it.
An inning is three outs, and baseball writes a part-finished inning as a figure that looks like a decimal and is not one: 5.2 means five innings and two outs, which is 5 + 2/3 = 5.667 innings. Dividing by 5.2 instead makes the rate about 9% too high at that length. This page therefore takes the innings and the outs as two separate fields, and the outs field cannot go above 2, because a third out finishes the inning and the figure rolls over. Baseball-Reference prints Sandy Alcantara’s 2022 workload as “228.2”, which is 228 innings and two outs. MLB’s glossary gives the rule behind the notation without ever stating the notation: “Because there are three outs in an inning, each out recorded represents one-third of an inning pitched.”
The size of that error is not a single percentage, and saying it is would be wrong. The absolute shortfall is fixed at 7/30 of an inning per out, so the proportional error depends entirely on how many innings it is divided into: the table above runs from 0.13% to 233% on that one fact. It is always an overstatement, and it vanishes only on a whole number of innings, which is why a calculator that gets this wrong still looks right whenever it is tested on a round figure. The cricket run rate calculator meets the identical trap over six balls rather than three outs.
This figure contains a judgement, not just a count. Rule 9.12 of the Official Baseball Rules leaves the error to the official scorer, and Rule 9.16 then has that same scorer reconstruct the inning as though the defence had played without errors or passed balls to decide which runs were earned. Two scorers can watch the same inning and file different numbers, and nothing in the arithmetic can recover the difference. ERA rests on a reconstruction of an inning that did not happen, made by one person at the ground. A pitcher in front of a poor defence collects unearned runs and keeps a flattering ERA, while the same runners are fully in the WHIP figure, which asks nothing about who dropped what. The fielding side of that judgement is the fielding percentage and range factor calculator, and cricket’s nearest equivalent to an ERA is the economy rate on the bowling economy, average and strike rate calculator.
Frequently asked questions
How do you calculate ERA?
Earned runs multiplied by nine and divided by innings pitched, under Rule 9.21(e). 3 earned runs in 9 1/3 innings is 2.89, the example the rule’s own Comment works. The nine is the innings in a complete game, so a seven-inning competition multiplies by seven.
What does 5.2 innings pitched mean?
Five innings and two outs, which is 5.667 innings. MLB’s glossary says each out “represents one-third of an inning pitched”, so the figure after the point only ever reads 0, 1 or 2. Dividing by 5.2 makes the ERA 8.97% too high.
Why is my ERA different from the one published?
Either the pitching line was divided as an ordinary decimal — always an overstatement — or the runs used were runs rather than EARNED runs. A run that scored through an error or a passed ball is unearned under Rule 9.16.
What makes a run unearned?
An error or a passed ball somewhere in the sequence that led to it. Rule 9.16 has the official scorer replay the inning as though the defence had played cleanly and decide which runs would still have scored. It is a judgement, not a count.
Is ERA the same as a cricket economy rate?
They answer the same question and share the same notation trap, but an economy rate is runs per over of six balls and counts every run, while an ERA is earned runs per nine innings and excludes the unearned ones.
Related calculators
References
- Official Baseball Rules, Rule 9.21, Determining Percentage Records — (a) winning percentage; (b) batting average, hits ÷ times at bat; (c) slugging percentage, total bases on safe hits ÷ at-bats; (d) fielding average, putouts plus assists ÷ total chances (putouts, assists, errors); (e) earned-run average, earned runs × 9 ÷ innings pitched, with a Comment working 3 earned runs in 9 1/3 innings as 2.89; (f) on-base percentage, hits plus walks plus hit-by-pitches ÷ at-bats plus walks plus hit-by-pitches plus sacrifice flies. Read on baseballrulesacademy.com, 12 Oct 2026: the official 2024 Edition PDF was fetched whole and its extracted text of 109,942 characters stops inside Rule 5.09, while its own contents page lists Rule 9.21 on page 141. Every formula taken from it is corroborated against MLB’s glossary.
- Official Baseball Rules, Rule 9.16, Earned Runs and Runs Allowed — “An earned run is a run for which a pitcher is held accountable”, charged only where it did not score through errors or passed balls; the official scorer reconstructs the inning as though the defence had played without them, a pitcher’s own error counts like any other, and the pitcher gets the benefit of the doubt. baseballrulesacademy.com (read 12 Oct 2026).
- Major League Baseball. Glossary: Earned Run Average (ERA) — ERA “represents the number of earned runs a pitcher allows per nine innings”, “with earned runs being any runs that scored without the aid of an error or a passed ball”, and “The formula for finding ERA is: 9 x earned runs / innings pitched.” It does not explain the 9. mlb.com (read 12 Oct 2026).
- Major League Baseball. Glossary: Innings Pitched (IP) — “Because there are three outs in an inning, each out recorded represents one-third of an inning pitched.” The entry does NOT state the “.1 and .2” scorebook notation, so this page demonstrates the notation from published lines instead. mlb.com (read 12 Oct 2026).
- National Federation of State High School Associations. NFHS Baseball Rules, Rule 4-2-1 — “A regulation interscholastic game consists of seven innings (turns at bat)”. Read on baseballrulesacademy.com, 12 Oct 2026; that reproduction carries no edition year, so the seven innings are cited as the NFHS’s standing rule rather than dated to a season.
- Potanko C. “No ghost runner, 7-inning doubleheaders and universal DH, oh my!” Pittsburgh Baseball Now, 11 March 2022 — “seven-inning doubleheaders will not return for the 2022 season”, the rule having been introduced for the shortened 2020 season and kept for 2021. Its further report that the extra-innings runner was ending is contradicted by MLB’s own February 2023 announcement, which is followed instead (read 12 Oct 2026).
- Major League Baseball. “Automatic runner permanent; new position-player pitching rules”, mlb.com, 13 February 2023 — the committee “voted on Monday to make two additions to the 2023 rulebook”; the automatic runner “was instituted in 2020 in an effort to preserve player health and safety during the pandemic-shortened season” and “applies only to regular-season games”. Read 12 Oct 2026.
- Baseball-Reference. Sandy Alcantara, standard pitching — the 2022 row reads W 14, L 9, ERA 2.28, IP “228.2” and WAR “8.0”. It corroborates the 8.0 bWAR quoted by the Samford article, and the printed “228.2” is a published instance of the notation this batch is built around: 228 innings and two outs. baseball-reference.com (read 12 Oct 2026).