No-Vig Calculator (Fair Odds)

How the no-vig calculator works

Bookmakers never offer truly fair odds — they shade both sides so the implied probabilities sum to more than 100%. That extra slice is the vig. Removing it reveals what the book actually thinks the fair probability is, which is the number you should be comparing against.

1. Convert each side's odds to an implied probability.
2. Add them together — the total will be over 100% (that's the overround).
3. Divide each side by that total to normalise it back to 100%.

Worked example

A game is priced −110 / −110. Each side implies 52.38%, totalling 104.76% — a 4.76% vig. Divide each by 1.0476 and you get a fair 50% / 50%. Now you know the book's true read, and you can line it up against the Polymarket price.

Using it with prediction markets

Because Polymarket and Kalshi run on razor-thin margins, their prices sit close to fair value. Strip the vig off a sportsbook line and compare: a meaningful gap can signal a mispriced market. Pair this with an odds converter to translate formats, or find scanners in the analytics tools directory that surface these gaps automatically.