The bookmaker's built-in commission. The difference between true odds and what you're offered.
Vig (short for vigorish, also called juice or margin) is the bookmaker's commission embedded in the odds. It's how sportsbooks guarantee profit regardless of outcomes.
In a fair market, a coin flip would be +100/+100 on both sides. Instead, books offer -110/-110, creating a 4.5% margin. You're effectively paying a tax on every bet.
De-vigging — removing the vig to find true probabilities — is essential for identifying value bets. Without knowing the true odds, you can't calculate expected value.
overround = sum of all implied probabilities - 1 vig per side varies by de-vig method (multiplicative, power, Shin, etc.)
At -110/-110: each side implies 52.38%. Sum = 104.76%. Overround = 4.76%. After multiplicative de-vig, each side's true probability = 50%.
The probability of an outcome implied by the odds. Includes the bookmaker's margin (vig).
OverroundThe total implied probability exceeding 100% across all outcomes in a market — the bookmaker's total margin.
True ProbabilityThe actual estimated likelihood of an outcome, with the bookmaker's margin removed.
Expected Value (EV)The average profit or loss per bet if you made the same wager thousands of times. Positive EV means long-term profit.