xwOBA (Expected Weighted On-Base Average) is Statcast's measure of what a hitter's wOBA should be based on the exit velocity and launch angle of every batted ball. It strips BABIP luck out of wOBA, making it the single most predictive hitter metric in modern baseball.
xwOBA uses Statcast's per-batted-ball data (exit velocity + launch angle) to assign each ball in play an expected outcome based on how similar batted balls have historically performed. A 105 mph line drive at 18° launch angle becomes a hit ~70% of the time historically, so it gets credited as ~0.7 of a hit value regardless of what actually happened (caught, beat the shift, dropped in).
The formula sums those expected outcomes per plate appearance and applies the same run-value weights as wOBA. Walks, HBP, and Ks count the same as in wOBA (no contact, no Statcast data). Only balls in play use the expected-value substitution.
wOBA is what happened. xwOBA is what should have happened. The difference is luck — the defensive plays, shift positioning, weak-grounder seeing-eye singles, and crushed liners that flew right at fielders. By stripping luck out, xwOBA predicts FUTURE wOBA better than past wOBA does.
A hitter running .280 wOBA but .340 xwOBA is hitting the ball well but getting unlucky. He'll regress UP. A hitter at .340 wOBA but .290 xwOBA has been lucky and will regress DOWN. The market is slow to adjust to xwOBA-vs-wOBA gaps; sharp bettors catch the regression early.
Same scale as wOBA:
For pitchers: opposing xwOBA vs opposing wOBA tells the same story in reverse. A pitcher allowing high xwOBA but low wOBA is getting bailed out by defense and will regress worse.
xwOBA is contact-quality only. It doesn't account for:
Lineup xwOBA vs handedness is a primary input. The model also tracks xwOBA-wOBA gaps per player to flag regression candidates. When a hitter's recent xwOBA is meaningfully above his recent wOBA, the model adjusts the expected runs upward for that side.
Theory is one thing — using the concept on tonight's slate is the value. These hubs are the practical application: