What a no-vig calculator does

Sportsbooks don't post true probabilities — they post odds that include a built-in margin called the vig (short for vigorish), also called juice. A standard -110/-110 two-way market looks like 50/50, but the implied probabilities actually sum to roughly 104.76%. That extra 4.76% is the book's take. A no-vig calculator strips that margin out and shows you what the market "really" thinks the probabilities are — the fair odds you'd need to break even in the long run.

How the math works

This calculator uses proportional devigging, the most common method. For each side of the market:

1. Convert the offered American odds to implied probability.
2. Sum the implied probabilities across all sides — the sum exceeds 100% by the vig.
3. Divide each side's implied probability by that sum to get its fair probability.
4. Convert the fair probability back to American odds.

Worked example: a -110/-110 moneyline. -110 has implied probability 110/210 = 52.38%. Both sides together sum to 104.76%. Dividing each by 1.0476 gives 50.0% fair probability for each side, which converts to +100 / +100 in American odds.

Proportional vs. Shin vs. multiplicative

Proportional devigging is the default because it's simple, close-enough for retail markets, and the method most public calculators use (so your numbers match what other tools show). Sharper tools use the Shin method, which assumes the vig is distributed unevenly — books build in more margin on the favorite than the underdog to protect against informed bettors. Shin is more accurate for sharp Pinnacle-style markets and less relevant for US retail books where the assumption mostly doesn't hold.

A third option, multiplicative devigging, treats the market as if both sides had their implied probabilities multiplied by the same constant. In practice it produces results very close to proportional for two-way markets and diverges slightly on three-way markets. We don't expose it because the difference rarely matters for real betting decisions.

When to use fair odds in your betting

Fair odds are what you should compare your own probability estimates against. If you think a team wins 55% of the time and the fair line is 50%, that's a 5-point edge — a bet you'd make. If the book is quoting -110 (implied 52.38%, fair 50%), you're getting +100 fair price on a 55% outcome: a clear +EV bet. If the book is quoting -130 (implied 56.5%, fair ~54%), the bet is negative EV even though your probability estimate was right about the direction.

Mongoose Bets' main methodology compares our sim-derived probabilities against vig-free fair odds exactly like the ones this calculator produces. See the methodology page for the full pipeline.

Common mistakes

• Using raw implied probability as fair probability. A -150 favorite is 60% implied, but fair is closer to 58% after devigging. Don't forget to remove the vig before calculating EV.
• Assuming symmetric vig. On a -110/-110 line, both sides eat the same vig. On a +150/-180 line, the vig is not evenly distributed (the Shin method models this). Proportional devigging gets you close, but sharp bettors cross-check.
• Devigging across different books. The right approach is to devig within a single book's two-sided market. Mixing the best price from Book A with the worst price from Book B and devigging the pair is meaningless — you're creating a synthetic market that neither book posted.