Vig (also called overround, juice, or margin) is the built‑in edge a sportsbook holds over bettors. If you convert both sides of a matchup into implied probabilities and add them together, the total will usually be more than 100%. That extra amount above 100% is the vig.

We convert American odds to implied probabilities for both sides using:
p = 100/(odds+100) for plus odds and p = -odds/(-odds+100) for minus odds. The vig is (p1 + p2) − 1. If that sum is exactly 1 (100%), the market would be “fair” with no house edge.

If you remove the vig proportionally from both sides so that the probabilities add to 100%, you get fair odds. Many bettors compare fair odds across books to find value or to estimate a true price before placing a wager.

Very sharp or off‑market prices can briefly add to ~100% or slightly less after rounding, especially on low‑liquidity props. In practice, sportsbooks keep margin, so sustained negative vig is rare and often a data or rounding artifact.