What Is FIP? FIP, xFIP and xERA Explained With Examples

ERA tells you how many runs scored. FIP, xFIP and xERA try to tell you how many runs a pitcher deserved, by leaving out things he does not control well. All three sit on the ERA scale, so a 3.50 FIP reads like a 3.50 ERA. Definitions below are from the FanGraphs Library and the MLB.com glossary.

FIP: Fielding Independent Pitching

FIP counts only outcomes that do not involve the fielders: strikeouts, walks, hit batters and home runs. FanGraphs' formula: FIP = ((13 × HR) + (3 × (BB + HBP)) − (2 × K)) ÷ IP + constant (FanGraphs: FIP, MLB glossary).

The constant exists only to put FIP on the ERA scale. It is set each season so that league-average FIP equals league-average ERA, and FanGraphs says it is generally around 3.10. It is computed as lgERA − ((13 × lgHR) + (3 × (lgBB + lgHBP)) − (2 × lgK)) ÷ lgIP, and FanGraphs publishes each year's value on its Guts! page.

The weights (13, 3 and −2) reflect roughly how many runs a home run, a walk and a strikeout are worth relative to a ball in play. Balls in play are left out entirely, because whether they turn into hits depends heavily on defense and luck.

xFIP: FIP with a league-average home run rate

Home runs per fly ball jump around from season to season for most pitchers. xFIP swaps a pitcher's actual home runs for the number he would have allowed at the league-average home-run-per-fly-ball rate (FanGraphs puts it between 9 and 10% depending on the year): xFIP = ((13 × (fly balls × lgHR/FB)) + (3 × (BB + HBP)) − (2 × K)) ÷ IP + constant (FanGraphs: xFIP).

xFIP is most useful for a pitcher whose home-run rate looks out of line with his past. It can mislead for pitchers who genuinely suppress or give up hard fly balls, and for anyone who pitches half his games in an extreme park.

xERA: Statcast expected ERA

MLB describes expected ERA as a 1:1 translation of expected weighted on-base average (xwOBA), converted to the ERA scale. xwOBA credits strikeouts, walks and hit batters as they happened and replaces each ball in play with the expected value of its exit velocity and launch angle (and, for some batted balls, sprint speed). So xERA is similar in spirit to FIP but also credits a pitcher for weak contact and charges him for hard contact, while removing park, weather and defense.

Worked example

Same made-up starter as in How to read pitcher stats: 180 IP, 190 strikeouts, 45 walks, 5 hit batters, 20 home runs, 160 fly balls, 3.00 ERA. We use a constant of 3.10, FanGraphs' typical value; a real calculation takes that season's constant.

xERA cannot be worked out by hand from a stat line; it needs pitch-by-pitch Statcast data, which Baseball Savant publishes for every pitcher.

What is a good FIP?

FanGraphs' rule-of-thumb scale (built on the 2016 run environment): about 3.20 excellent, 3.50 great, 3.80 above average, 4.20 average, 4.40 below average, 4.70 poor and 5.00 awful. Because FIP is pinned to league ERA, the right comparison is always that season's league average.

Which one should you use?

BullpenEdge's probable pitcher pages and the game projections use strikeout and walk rates for the same reason FIP does: they are the parts of pitching that repeat.