What Is ERA in Baseball? Earned Run Average Explained, Plus OPS

ERA, short for earned run average, tells you how many earned runs a pitcher gives up for every nine innings pitched. You find it by taking the pitcher’s earned runs, multiplying by 9, and dividing by the innings that pitcher has thrown. MLB’s glossary calls ERA “the most commonly accepted statistical tool for evaluating pitchers.”

The Official ERA Formula

MLB’s Official Baseball Rules set out the calculation in the rule on percentage records. The steps are: take every earned run charged to the pitcher, multiply that number by 9, and then divide by innings pitched, counting any partial innings.

ERA = 9 x Earned Runs / Innings Pitched

The rulebook includes a sample case. Three earned runs over 9 and one-third innings works out to an ERA of 2.89. MLB’s glossary uses the same formula.

A Worked Example

Take a pitcher who has been charged with 12 earned runs across 45 innings.

Earned runs times 9: 12 x 9 = 108.
Divide by innings: 108 / 45 = 2.40.

So the ERA is 2.40, or 2.4 earned runs for every nine innings.

An Example With a Partial Inning

Now take 7 earned runs over 20 and two-thirds innings.

Earned runs times 9: 7 x 9 = 63.
Innings as a decimal: 20 and two-thirds is about 20.667.
Divide: 63 / 20.667 is about 3.05.

Why the Number 9?

Under the rulebook, a regulation game is nine innings long, though a tie can extend it and some games end early. ERA uses that same figure as its multiplier.

What Makes a Run “Earned”

Everything in ERA depends on which runs count as earned. In the rulebook’s words, an earned run is one “for which a pitcher is held accountable.” To sort earned runs from unearned ones, the official scorer pictures the inning again with the errors and passed balls removed, and gives the pitcher the benefit of the doubt about where runners would have ended up.

Under the rules, a run goes on the pitcher’s earned record when the runner reached home through things like:

Hits, home runs included.
Walks and hit batters.
Sacrifice bunts, sacrifice flies, stolen bases, putouts and fielder’s choices.
Wild pitches (which the rules call solely the pitcher’s fault) and balks.

A run is unearned when it comes about only because of a fielding miscue. The rulebook walks through a case like this one. With two outs, a batter gets on through an error, and then the next two hitters both homer. Three runs cross the plate, yet none are earned, since without the error the inning would have been over.

A few further points from the rules:

When working out earned runs, an intentional walk is handled the same as any other walk.
A batter who gets on through catcher’s interference does not produce an earned run if they later come around to score.
If a pitcher is replaced in the middle of an inning, the reliever is not charged with runs scored by runners who were already on base when the reliever came in.

OPS: The Hitting Side

OPS stands for on-base plus slugging. MLB’s glossary explains it as on-base percentage and slugging percentage added into a single figure, meant to bring together a hitter’s ability to get on base and to hit for average and power.

OPS = OBP + SLG

The Two Parts

On-base percentage (OBP): under the rulebook, add hits, walks and times hit by pitch, then divide by the total of at-bats, walks, times hit by pitch and sacrifice flies.
Slugging percentage (SLG): the bases a hitter gets per at-bat. A single counts once, a double twice, a triple three times and a home run four times, and that total is divided by at-bats.

An OPS Example

Say a hitter has an OBP of .365 and a slugging percentage of .470. Adding them gives an OPS of .835.

OPS Against

MLB’s glossary notes that OPS is also used when looking at pitchers. In that form it is known as OPS against.

ERA and OPS Side by Side

ERA is a pitching stat built on earned runs and innings pitched.
OPS is a hitting stat built on on-base percentage and slugging percentage.
ERA leaves out runs that scored only because of errors or passed balls.
OPS combines how often a batter reaches base with how many bases their hits are worth.

Frequently Asked Questions

Is a lower ERA better?

Yes. ERA counts earned runs per nine innings, so a lower figure means fewer earned runs were charged to the pitcher for the innings they threw.

Do unearned runs count in ERA?

No. ERA is built only from earned runs. Runs the official scorer rules unearned, because they depended on errors or passed balls, stay out of the calculation.

What does OPS stand for?

OPS stands for on-base plus slugging. You get it by adding a hitter’s on-base percentage to their slugging percentage.

What Is ERA in Baseball? Earned Run Average Explained, Plus OPS