Win Probability Calculator

Home Advantage is only added to Player/Team A's effective rating for the purpose of the win-probability formula and the resulting rating update — the Player/Team A Rating field you entered is treated as their true underlying rating and isn't overwritten. This mirrors how Elo systems typically model home-field edge as a temporary adjustment for one specific match, not a permanent change to the competitor's rating.

K-Factor only affects the New Rating and Rating Change outputs, not Win Probability — probability depends purely on the rating gap (plus home advantage), while K scales how far the post-match rating moves in response to the actual result you select. A K of 32 means a single upset win against a much higher-rated opponent can swing a rating by nearly 32 points, while a low K-factor of 10 (common in established chess ratings) produces much smaller, more gradual adjustments.

Elo is a zero-sum rating system by design — whatever points one competitor gains from an unexpected result are approximately what the other loses, since expectedB is calculated as 1 minus expectedA. The two rating changes won't be exactly equal in magnitude unless expectedA and expectedB are calculated from the identical rating gap (which is the case here), so in this calculator's model they are mirror images of each other.

Implied Decimal Odds is simply 1 divided by the calculated win probability for Player/Team A, so a value of 2.00 corresponds to exactly a 50% win probability (a break-even bet with no house edge). Lower decimal odds mean a stronger favorite according to the Elo ratings entered; comparing this number to real bookmaker odds can indicate whether the market sees the matchup differently than a pure rating-based model does.