What is WPA (Win Probability Added) in baseball?

Win probability added — WPA — takes the idea of win probability and turns it into a way to credit individual players. If win expectancy is the team’s chance of winning at a given instant, WPA is the amount a single play changed that chance, handed to the player responsible.

Every plate appearance moves the team’s win probability, up or down. WPA simply captures that movement and assigns it. A batter who drives in the go-ahead run gets credited with the jump; a batter who grounds into a rally-killing double play gets debited for the drop. The pitcher on the other side receives the mirror image.

Because it’s built directly on win expectancy, WPA measures something traditional stats can’t: not just whether a player did something good, but how much that something actually mattered to winning. If you haven’t yet, it’s worth understanding how win probability works first — WPA is impossible without it.

For all its power, the math is almost trivial: WPA equals the team’s win probability after a play minus its win probability before. That’s the whole formula.

Say a team’s chance of winning is 45% when a batter steps in. He launches a home run, and the number jumps to 60%. His WPA for that plate appearance is 0.60 minus 0.45, or +0.15 — he added fifteen percentage points of winning to his team. The pitcher who served it up gets -0.15.

Add up everything a player does across a game and you get his total WPA for the day; the hero of a walk-off win might finish north of +0.40. One useful anchor: a play, or a player, with a WPA of exactly 0 was perfectly average — it left the team’s odds of winning exactly where they were.

Here’s what makes WPA feel so different from a box score: the exact same hit can be worth wildly different amounts depending on when it happens. A solo home run that ties the game in the ninth might be worth +0.30 WPA. The identical home run in a 10–1 blowout might be worth 0.02 — technically it helped, but the game was already decided, so it barely moved the odds.

This is leverage doing its work. Because WPA is the change in win probability, and win probability swings hardest in high-leverage moments, WPA automatically rewards players for coming through when it counts.

If you want the full picture of why some moments carry so much more weight, leverage index is the companion idea. Together they explain why a quiet single can outweigh a loud, meaningless one.

Single plays are fun, but WPA is at its best summed up. Total every win-probability swing a hitter caused across all his plate appearances and you get his contribution to that specific game — a clean answer to “who actually won this?” that the box score often gets wrong.

Because it captures everything that changes win probability, WPA credits more than just hits: a productive walk in a big spot, a steal that improves the odds, a sacrifice that moves the winning run over — all of it shows up. And because 0 marks a league-average play, the sign tells you the direction at a glance: positive players helped their team win, negative players hurt it.

One honest caveat worth keeping: WPA is a story of what already happened, a brilliant summary of a game’s swings — it isn’t a forecast of future skill.

The clearest way to feel WPA’s value is a head-to-head with old-school stats. Imagine one hitter goes 4-for-4 with a couple of runs — but every hit lands in a 9–2 rout, after the outcome was settled. The box score adores him. His WPA is nearly nothing, because none of it changed whether his team won.

Now imagine a teammate who goes 1-for-4, but that single hit is a two-out, two-run single in the bottom of the ninth to win the game. The box score barely notices. His WPA is enormous.

WPA flips the traditional hierarchy to reward impact over accumulation — the timing of production, not just the volume of it. That’s exactly the blind spot it was designed to fix, and it’s why a single number can tell you who truly carried a team on a given night.

WPA is powerful but tedious to compute by hand — you’d need the win probability before and after every plate appearance. TwentySeven does it for you. Every game includes Player Impact rankings that rate each batter by the sum of the win-probability changes they caused, which is WPA by another name.

The payoff is a lineup sorted by who actually moved the game. A hitter who came up big in high-leverage spots ranks above one who padded his line in a blowout — exactly the reordering WPA is meant to produce. Pair it with the game’s win-probability curve and Top Moments panel and you can trace not just how a game swung, but who swung it.

Start with how win probability works and leverage index to see the full system these rankings are built on.