What is WPA in League of Legends?
WPA (win probability added) is how far a play changed a team's win probability in a League of Legends game. Every kill, death, objective and item bought receives one, positive when it helped the team and negative when it hurt.
How it's calculated
A team's win probability (WP) is how often teams in similar game states went on to win in the end. For the same gold, XP, towers, dragons, players alive and minute of the game. Mentorr's model, Flow, calculates that percentage having learned from 4,800,000 ranked games, reading each one second by second.
A play's WPA is one subtraction:
WPA = WP after the play − WP before it
The minute affects the price. 1,000 gold at 8:00 is worth more than at 35:00, and a dead enemy is worth more at 30:00 than at 5:00, so the same kill will get a different WPA at different points of the game.
An example from a ranked game
Galio against Ekko mid in Platinum. At 1:34, Ekko killed Galio. Galio's team had 36.0% WP just before the kill and 32.1% just after, so the death cost −3.9 WPA.
Flow cannot know for certain why the kill happened, but it prices what the kill changed: the gold and XP Ekko got, and what Galio lost on the map until the respawn.
Winrate, WP, and WPA
A winrate counts games won, WP estimates wins, and WPA tells you how a play changed the win probability.
| Winrate | WP | WPA | |
|---|---|---|---|
| Counts | Games won, out of games played | How likely a team is to win (at one point in the game) | How much one play moved a team's win probability |
| Written as | A percentage of games | A percentage 0-100% | A signed amount: −3.9 WPA |
| Tells you | How often a champion, an item or a player wins | Which team is currently "winning" | Which plays, players, items, champions helped win or lose a game |
You can have a good winrate on a champion, but still have a negative WPA in the games you play it. Your winrate includes all of your teammates' efforts, but your WPA carries only your own contribution.
How accurate it is
On 152,400 ranked games it never saw, teams won about as often as Flow said, off by 0.49 points on average: when it says 70%, those teams won between 69.5% and 70.5% of the time.
It's weakest early, since not enough has happened yet. In the first ten minutes of a game it picks the winner 73 times in 100, against 85 to 91 after. That being said, a WPA on a play from the first minutes is still the right price for what changed (so your early plays are still priced very accurately).