Things You'll Need:
- Statistics on the Pitcher
- Pen
- Paper
- Calculator
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Step 1
The pitcher's earned run average is simply an estimate of how many runs s/he would give up if s/he pitched a full nine inning game. (You can convert everything to softball format by using 7 in place of 9.) This is calculated by dividing all the earned runs a pitcher has given up by the number of innings pitched and then multiplying the result by 9. So if a pitcher has given up 12 runs and has pitched 12 innings, that pitcher has an ERA of 12/12*9= 9.00. If a pitcher has given up 12 runs but has pitched 36 innings, that pitcher has an ERA of 12/36*9= 3.00.
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Step 2
The first thing to be aware of when calculating the ERA is that each out counts as 1/3 of another inning, and any men left on base when the pitcher is taken out count against that pitcher. So if a pitcher is taken out after one out in the 5th inning and has allowed no runs but the bases are loaded, and the new pitcher gives up a grandslam home run, the old pitcher pitched 4 1/3 innings and gave up 3 earned runs, for an ERA of 3/4.333*9= 6.23. Assuming the new pitcher gets the next two batters out and then is taken out of the game, he has only given up 1 run in 2/3 innings, for an ERA of 1/.667*9= 13.5.
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Step 3
The second thing to be aware of is the expression, "earned run." An earned run is any run that is scored due to the pitcher's pitching. If a runner reaches base because of a hit, a walk, or getting hit by a pitch, if that runner reaches home, that counts as an earned run. If a runner is on third and reaches home on a wild pitch, that counts as an earned run. What does NOT count as an earned run is any hitter who reaches base on an error by a fielder. If the shortstop bobbles the ball and the hitter reaches first when it would have been an out, that runner does NOT count in the pitcher's ERA. It is entirely possible (though unlikely) for a pitcher to lose a 3-0 game and finish with an ERA for the game of 0 if the only runners to score reached base on fielding errors. So be careful to eliminate those scores from your pitcher's ERA.
