How this calculator works
FIP starts from the idea that a pitcher's results on balls in play are heavily influenced by defence and luck. It replaces hits allowed with the events a pitcher controls most directly. Home runs are weighted 13, walks and hit batters 3, and strikeouts minus 2, all divided by innings pitched. The league constant shifts the whole scale so that league-average FIP equals league-average ERA. FIP is not a prediction of ERA for a single season, and it ignores defence, park effects and the quality of contact allowed.
Formula
FIP = (13 x HR + 3 x (BB + HBP) - 2 x K) / IP + C. The constant is C = league ERA - (13 x league HR + 3 x (league BB + league HBP) - 2 x league K) / league IP.
Worked example
A pitcher allows 10 home runs, 20 walks and no hit batters, strikes out 100 and records 300 outs (100 innings). The run total is 130 + 60 - 200 = -10, or -0.10 per inning. With a constant of 3.00 the FIP is 2.90. For a league with a 4.00 ERA, 5,000 home runs, 14,000 walks, 1,800 hit batters, 40,000 strikeouts and 43,000 innings, the constant is 4.00 - 0.753 = 3.247.
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Frequently asked questions
Why is a strikeout worth -2?
Strikeouts remove the chance of a hit or an out being converted into a run on a ball in play. The -2 weight balances the formula against an average pitcher's other events so that the league constant lands near 3.1 to 3.2.
Does FIP replace ERA?
No. ERA records what happened. FIP estimates the part of run prevention that a pitcher controls. A large gap between them usually reflects defence, park or luck, and it tends to shrink over several seasons.
Which constant should I use?
Use the constant published for your league and season, or compute it from league totals in the constant mode. The constant changes as the run environment changes, so a constant from another era will shift the scale.