How this calculator works
Earned runs are runs that score without the help of an error or a passed ball, as judged by the official scorer. Innings pitched are counted in outs: every three outs make one inning, so six innings and two outs is 6.667 innings. The regulation length is normally nine innings, and the constant 9 in the formula follows that length. A seven-inning league uses 7, which lowers the ERA for the same rate of run prevention. Innings pitched is never rounded to a whole number before the division, because that would understate a partial inning.
Formula
ERA = (earned runs x innings per game) / innings pitched, where innings pitched = complete innings + additional outs / 3. Rearranged: earned runs = ERA x innings pitched / innings per game; innings pitched = innings per game x earned runs / ERA.
Worked example
A pitcher allows 3 earned runs over 6 complete innings and 2 outs. Innings pitched is 6 + 2/3 = 6.667. ERA = 3 x 9 / 6.667 = 4.05. Four earned runs over nine innings gives 4 x 9 / 9 = 4.00.
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Frequently asked questions
Why does 6.2 innings mean 6 and two thirds?
Baseball writes the number of outs after the decimal point. The 2 means two outs, not two tenths of an inning, so the decimal form is 6 + 2/3.
Do unearned runs count?
No. ERA counts only earned runs. Runs that follow an error or other scorer ruling are unearned, although they still count against the pitcher's runs allowed total.
How do I use a 7-inning game?
Choose the 7-inning option. The same line of 3 earned runs in 6.667 innings then gives 3 x 7 / 6.667 = 3.15 rather than 4.05.