Welcome to Homerunweb, a website about baseball statistics, book reviews, and more by David H. Martinez, SABR member and author of The Book of Baseball Literacy: 3rd Edition.

How to Calculate Earned Run Average (ERA)

As the basic statistical measure of a pitcher, ERA determines the average number of earned runs scored against a pitcher every nine innings; an earned run is a run that’s the pitcher’s fault, while an unearned run, which is usually the result of a fielding error, is not.

How to calculate ERA? The formula is simple, but you do need a calculator:

Earned Runs x 9 / Innings Pitched

So, for example, a pitcher with a 3.50 ERA is expected to allow three and a half earned runs whenever he pitches a complete game. To be eligible for the yearly ERA championship, a pitcher must have thrown at least one inning for every game his team played, usually 162.

From a historical perspective, it’s important to realize that baseball has changed so much that any listing of the top single-season and career leaders in ERA has no meaning. From 1876 through 1892, when the pitching box stood at between 45 and 50 feet from home, and again from the late 1890s through the 1910s, top ERAs remained in the mid-1.00s and an ERA over 3.00 was poor. In the lively ball era, from 1920 through World War II, an ERA below 2.00 was a rare and phenomenal event. From the war until the 1960s, ERAs fell slightly, and in the mid- to late-1960s, they reached lows that hadn’t been seen since the dead ball era. From 1969 until about 1993, ERAs were pretty stable, with the league-leader occasionally below 2.00 and the league average around the mid 3.00s.

Over the last decade, we’ve been in sort of a new lively ball era and only a few pitchers post ERAs in the low 2.00s; the league average now is usually around 4.50.

Visit Baseball-Reference.com for complete leaders in ERA: single-seasoncareeractive.

The Book of Baseball Literacy: 3rd Edition

For hundreds of descriptions, stats, and stories like this, read The Book of Baseball Literacy: 3rd Edition, now available as an ebook exclusively for Amazon Kindle for just $2.99 and in a print edition for just $14.95. Learn more about the book