How do you calculate ERA?

How do you calculate ERA? The formula for finding ERA is: 9 x earned runs / innings pitched. If a pitcher exits a game with runners on base, any earned runs scored by those runners will count against him. ERA should be an ideal evaluation of pitchers.

The formula for finding ERA is: 9 x earned runs / innings pitched. If a pitcher exits a game with runners on base, any earned runs scored by those runners will count against him. ERA should be an ideal evaluation of pitchers.
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