Calculating Basic Baseball Stats - Earned Run Average (ERA)

If you're a baseball fan, you probably are aware of how important statistics are to the game. A casual fan probably understands what most of the most popular statistics measure, in a general sense, but they may not know how to calculate these stats for themselves. Let's look at a one of the most basic pitching statistics and see how it's calculated.

Earned Run Average, or ERA, is one of the oldest and most well known pitching statistics. It originally appeared in the early 1900's, and has been used extensively since. It remains one of the most accurate measures of a pitcher's performance (especially for starting pitchers). ERA denotes the number of Earned Runs a pitcher gives up per nine innings (the length of a regulation game).

What makes a run unearned? When a run is scored because of (or by a runner on base because of) an error, passed ball, interference, or muffed fly ball that prolongs an at bat in which a hitter eventually gets on base, it is considered unearned. These runs are ignored when looking at ERA, as they can be presumed to be the fault of another fielder, not the pitcher.

To calculate Earned Run Average, use the following formula:

ERA = (Earned Runs x 9) / Innings Pitched (IP)

ERA is denoted as a number followed by a decimal point with two numbers after the decimal point. For example, a pitcher who allows 3 earned runs in 8 innings pitched would have an ERA of 3.38 (3 ER x 9 / 8).

In 2005, the "league average" ERA in the National League was 4.45, and in the American League it was 4.76. The American League total is typically a bit higher because they use the DH, while in the National League, pitchers hit for themselves. A pitcher who is better than league average by a substantial margin is probably a pretty good pitcher.

In 2005, Roger Clemens of the Houston Astros led the National League in ERA with an amazing 1.87 mark. Clemens allowed 44 Earned Runs (out of 51 total runs allowed) in 211.3 innings pitched. It marked the 7th time that Clemens had led his league in ERA, an impressive addition to a resume that will surely land him in the Hall of Fame when he becomes eligible. Clemens career ERA is 3.12.

In the American League, Kevin Millwood of the Cleveland Indians led the league in 2005 with an ERA of 2.86. He allowed 61 Earned Runs (72 totals runs) in 192.0 IP. This marked Millwood's first ERA title, although he finished second in 1999 when he turned in a 2.68 ERA for the Atlanta Braves.

Both Clemens and Millwood are an excellent example of why ERA is important in determining the value of a pitcher. Neither pitcher got good run support from their teams, so if you looked at just their win loss records, you would have no idea how well they pitcher. For Millwood's league leading ERA, his record was only 9-11. Clemens faired slightly better, at 13-8, but with his ERA, with a bit better run support, he could have potentially won 20 games and gotten more support in Cy Young Award balloting.

The lowest qualifying (minimum of 1 IP per team game) ERA since 1900 belongs to Dutch Leonard, who turned in an amazing 0.96 ERA in 1914 for the Boston Red Sox. Leonard allowed only 24 Earned runs in 224.6 innings. The lowest ERA of the post-WWII era belongs to Bob Gibson, who turned in a 1.12 mark in 1968 for the St. Louis Cardinals.

The lowest career ERA for a pitcher with a minimum of 1,000 career innings pitcher belongs to Ed Walsh, who pitcher from 1904 to 1917. His career mark was 1.82. Because of changes in the game over the years, many of the career leaders are from the time around the turn of the century to about 1920.

ERA is a useful stat to compare pitchers to their peers, but it alone is not always an accurate way to compare pitchers across eras. Look into ERA+ for a way to better compare pitchers across history.