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See ERA Calculator here>>http://www.miniwebtool.com/era-calculator/

A major league pitcher is often judged on the basis of his earned run average, or ERA. This number represents the average number of earned runs given up by the pitcher per nine innings.

An earned run is any run that the opponent scores off a particular pitcher except for runs scored as a result of errors. For instance, if Tim Lincecum gives up three solo home runs, and then an error causes another run to score, he is only credited with those first three runs that were "his fault."

The earned run average can be calculated using the following formula:

(Earned Runs/Innings Pitched) x 9

Therefore, if Roy Halladay is charged with 19 earned runs in his first 89 innings pitched, his ERA would be 19 divided by 89, which is .2135, times 9, which is 1.92, a very good number.

(19 runs / 89 innings) x 9 = 1.92

Don't forget the 9 at the end. By calculating runs/innings you have only figured out earned runs per inning, but you must keep in mind that an ERA is actually earned runs per nine innings, since a regulation game is 9 innings. The number, usually represented with two places after the decimal, shows how many runs the pitcher gives up in an average complete game.

Here's one last example: Johan Santana yielded 66 earned runs over 234.33 innings in 2008. What is his ERA? Simple -- divide 66 runs by 234.33 innings and multiply by 9. The correct answer is 2.53.

http://www.freemathhelp.com/earned-run-average.html

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-season, career, active.

http://www.homerunweb.com/era.html

Good info!!  I never could figure that out either.  Some things about baseball we baseball fand are too embarrassed to ask.