Implied probability converter: turn any odds or cent price into a probability, then take the vig out
A sportsbook quotes −110, a European site quotes 1.91, a British one 10/11 and Kalshi 52 cents, and all four are statements about the same thing: how likely the outcome is, plus a cut for the house. This converter puts them on one scale. Enter the prices for every outcome in a market, in American, decimal or fractional odds or in cents, and it returns the implied probability of each, the overround the prices add up to, and the no-vig fair probability with the matching fair price in every format. Enter one price and it works as a plain converter; enter both sides of a market and it shows what the margin is costing you.
Implied probability = 1 ÷ decimal odds. Overround = sum of implied probabilities. No-vig probability = implied probability ÷ overround. Cents are read as the price of a contract that pays $1. Fees and the bid-ask spread on exchanges are not included.
How it works
Every price converts to decimal odds first, because decimal odds are the payout per unit staked including the stake, and their inverse is the implied probability. American odds of +150 become 1 + 150/100 = 2.50; −110 becomes 1 + 100/110 = 1.909; fractional 6/4 becomes 1 + 6/4 = 2.50; a 40-cent contract that pays $1 becomes 1 ÷ 0.40 = 2.50. The implied probabilities of all the outcomes in a market should add to 100%. On a sportsbook they add to more, and the excess is the margin. The converter divides each implied probability by the total, which removes the margin in proportion to each price, the method most published no-vig figures use.
| Market | Prices | Implied probabilities | Overround | No-vig probabilities |
|---|---|---|---|---|
| Standard spread | −110 / −110 | 52.38% / 52.38% | 104.76% | 50.0% / 50.0% |
| Moneyline favourite | −200 / +170 | 66.67% / 37.04% | 103.70% | 64.3% / 35.7% |
| Soccer 1X2 | 2.10 / 3.40 / 3.80 | 47.62% / 29.41% / 26.32% | 103.35% | 46.1% / 28.5% / 25.5% |
| Exchange, both asks | 52¢ / 50¢ | 52.00% / 50.00% | 102.00% | 51.0% / 49.0% |
The last row is the one sportsbook bettors tend not to expect. An exchange has no vig in the bookmaker's sense, but if you buy at the asking price on both sides the two prices add up to more than $1, and the excess, the bid-ask spread, works exactly like an overround. On a liquid contract it is often one or two cents; on a thin one it can be far wider than any sportsbook margin.
A worked example
A sportsbook has a game at −200 on the favourite and +170 on the underdog, and the same game trades on an exchange where the underdog's Yes contract costs 38 cents. Entered as American odds, the sportsbook prices imply 66.67% and 37.04%, an overround of 103.70%. Taking the margin out leaves 64.3% for the favourite and 35.7% for the underdog, fair odds of about −180 and +180. The exchange asks 38 cents for something the sportsbook's own no-vig price values at 35.7 cents. Before fees, the sportsbook's +170 is the better price on the underdog: it pays as if the probability were 37.0%, while the exchange charges 38.0%. A Kalshi taker fee of 0.07 × 0.38 × 0.62, about 1.6 cents, would widen that gap further. Switch the input format to cents and enter 38 and 64 to see how the exchange market itself is priced.
Limits
Proportional margin removal assumes the bookmaker spread its margin evenly. In practice books often load more margin onto longshots, the favourite-longshot bias, so the proportional method can overstate an underdog's fair probability slightly; other methods, such as the power or Shin approaches, adjust for that and give somewhat different answers on lopsided markets. The converter needs every outcome of a market to remove the margin correctly: enter only one side and the overround cannot be measured. It does not include exchange fees, which you can price with the prediction market fee calculator, or the bid-ask spread unless you enter both asks. A no-vig probability is the market's estimate, not the true chance of the event. The tool does arithmetic on the numbers you enter; it is not a recommendation to bet or trade, and access to sportsbooks and prediction markets depends on your state.
FAQ
- How do you convert American odds to implied probability?
- For positive odds, divide 100 by the odds plus 100: +150 is 100 / 250 = 40%. For negative odds, divide the odds (as a positive number) by the odds plus 100: −150 is 150 / 250 = 60%. For decimal odds, implied probability is 1 divided by the odds.
- What is a no-vig probability?
- The implied probabilities of every outcome in a sportsbook market add up to more than 100%. The excess is the margin, or overround. Dividing each implied probability by the total removes it proportionally and leaves fair probabilities that sum to 100%. A −110/−110 market totals 104.76%, so each side is 50% after the vig is removed.
- How do Kalshi and Polymarket prices convert to odds?
- A contract that pays $1 and costs 40 cents implies a 40% probability, the same as decimal odds of 2.50 or American odds of +150, before fees. Fees raise the effective price slightly, so the probability you need to break even is a little higher than the quoted price.
- Why do the Yes and No prices on an exchange add up to more than $1?
- Buying at the ask on both sides costs more than $1 because of the bid-ask spread. Entered into the converter as two outcomes, the excess shows up as an overround, the exchange equivalent of a sportsbook's vig.
Related
- Implied probability explained: odds to percent, vig out
- Closing line value explained: how to measure CLV
- What is the vig? How sportsbook margin works
- Odds converter: American, decimal, fractional and implied probability
- Sportsbook vs exchange price comparison tool
- Is Kalshi legal in your state? The 50-state tracker
Sources
- Kalshi, Fee schedule, for the taker fee used in the worked example.
- Joseph Buchdahl, Football-Data, The Efficiency of the Pinnacle.com Closing Line, on margins in bookmaker prices.
- BetG8, Implied probability explained and What is the vig?, for the conversion formulas. Table figures are BetG8 calculations.
18+. This tool performs arithmetic on numbers you enter. It is not legal, tax, financial or betting advice. If gambling stops being fun, help is available at 1-800-GAMBLER.