How to remove vig from a market

Start with all outcomes

A valid two-way calculation needs both mutually exclusive sides of the same market at the same time. Convert each price to raw implied probability. At -120 and +100, the raw probabilities are 54.55% and 50.00%, for a 104.55% total.

Proportional normalization

Divide each raw probability by the total. Side A becomes 54.55 / 104.55 = 52.17%; Side B becomes 50 / 104.55 = 47.83%. They now sum to 100%. The corresponding no-vig decimal prices are approximately 1.92 and 2.09.

This is the method used by SimTheGame's public no-vig calculator because it is simple, explainable, and easy to reproduce.

Why the method matters

Proportional normalization spreads the margin according to raw probability. That is an assumption, not a revealed fact about how the book built the line. Additive, power, and odds-ratio methods distribute the overround differently. Differences become more material in long-shot markets and markets with many outcomes.

Data hygiene before mathematics

• Use one sportsbook or a clearly defined consensus source.
• Capture all sides simultaneously; do not pair a stale favorite with a current underdog.
• Match rules for overtime, pushes, voids, and participant eligibility.
• Do not normalize an incomplete outcome set.
• Preserve the original displayed prices for auditability.

What no-vig probability can tell you

It is a cleaner expression of the market's relative weighting. It can be used as an input to a market-implied simulation or as a baseline for comparison. It cannot, by itself, establish true probability, expected profit, or independence between related markets. A reproducible analysis says which de-vigging method it used and does not hide that choice.