GUIDES · BETSLIP DOCTOR
Why can ERA, FIP and xFIP describe the same pitcher differently?
ERA uses earned runs per inning. FIP focuses on home runs, walks, hit-by-pitches and strikeouts, with a league-alignment constant; xFIP replaces actual home runs with an estimate based on fly balls and the season's league home-run rate. Different inputs can produce different values.
For Bettors and cappers learning to read baseball records · Updated
ERA follows earned-run scoring
ERA is nine times earned runs divided by innings pitched. Its numerator depends on the official scorer's earned-run decisions, including how errors and passed balls affect an inning.
That makes ERA a record of earned runs allowed, not a tally of every run or a guarantee of future results.
FIP and xFIP use different event inputs
MLB's FIP formula uses home runs, walks, hit-by-pitches, strikeouts and innings pitched, then adds a FIP constant to place the result on the league ERA scale. It leaves results on balls hit into the field out of the calculation.
xFIP keeps the related framework but estimates home runs from a pitcher's fly balls and the league's home-run-per-fly-ball rate for that season. The season rate and FIP constant are inputs, not universal fixed values.
Fictional component ledger
Fictional line: 6 IP, 2 earned runs, 1 home run, 2 walks, 1 hit-by-pitch, 6 strikeouts and 10 fly balls. ERA can be calculated as 9 × 2 ÷ 6 = 3.00. FIP would use the actual home run and the walk, hit-by-pitch, strikeout and inning totals plus that season's FIP constant. xFIP would replace the actual home-run input with an estimate from 10 fly balls and the season's league HR/FB rate. Since this fictional ledger supplies neither season-specific constant nor league rate, no FIP or xFIP total is calculated.
Related questions
Does FIP count hits on balls in play?
MLB's FIP definition removes results on balls hit into the field and focuses on strikeouts, walks, hit-by-pitches and home runs.
Does xFIP use a pitcher's actual home-run total?
It substitutes an estimate based on fly balls and that season's league-average home-run-per-fly-ball rate.