This implied probability calculator turns a posted price into the probability it implies. Enter one side for the raw conversion, or enter both sides of the market and it shows you the book's margin (the vig) and the fair, no-vig probability for each side. Every line of arithmetic is shown so you can check it by hand.
Implied probability is the win probability baked into a posted price. A favourite at -150 needs to win about 60% of the time for that price to break even, and the conversion is simple enough: 150 divided by (150 + 100). For an underdog at +130, it's 100 divided by (130 + 100), about 43.5%. Decimal odds are simpler still: divide 1 by the price.
That 60% isn't the book's honest estimate of the favourite's chances. Add both sides of the same market together and you get about 103.5%, not 100%. A fair market sums to exactly 100%, so the extra 3.5% is margin the book built into the prices. It's called the overround, or the vig, and it's how the book profits no matter which side wins. Any implied probability calculator that returns one number and stops is quietly handing you a figure inflated by that margin.
The number an actual decision should use is the fair, no-vig probability: each side divided by the book total, so the two sides sum back to 100%. For -150 and +130, that's 60% and 43.5% each divided by 103.5%, giving about 58.0% and 42.0%. If your own estimate of the favourite is higher than 58%, that's the gap between your read and the market. If it's lower, the gap runs the other way. Either way that's a read on the number, not a recommendation to act on it. The raw 60% can't show you that gap at all, because part of it's margin.
That limit belongs here rather than buried further down the page. This implied probability calculator is a maths tool. It explains how a posted price converts to a probability and what the margin costs. It doesn't tell you which side wins, and nothing on this page is a recommendation to stake money on anything.
Say a market posts -150 on one side and +130 on the other. Convert the favourite: 150 / (150 + 100) = 60.0%. Convert the underdog: 100 / (130 + 100) = 43.5%. Sum them: 60.0% + 43.5% = 103.5%. Subtract 100% and the vig is 3.5%, which is what the book keeps for dealing the market. Then strip it: 60.0% / 103.5% = 58.0% fair on the favourite, and 43.5% / 103.5% = 42.0% fair on the underdog. The pair sums to 100%, as a fair market must.
A standard -110/-110 market tells the same story with a bigger margin: each side converts to 52.4%, the total is 104.8%, and the vig is about 4.8%. That means a coin-flip price asks you to win 52.4% of the time to break even, not 50%. The vig is the quiet tax on every price, and seeing it spelled out is worth more than any single converted number.
One more limit worth naming, on top of the one above. Stripping the vig tells you what the market's price implies once the margin is removed. It doesn't tell you the market is right. The fair number is still the market's opinion, just an undistorted version of it.
Implied probability is the win probability baked into a posted price. American odds of -150 convert to 60% (150 divided by 250), and +130 converts to about 43.5% (100 divided by 230). Decimal odds are even simpler: divide 1 by the price, so 1.91 becomes about 52.4%. It tells you what the price says, not what will happen.
Because the book builds a margin into the prices. If both sides were fair, they would sum to exactly 100%. Real books post something like -150 and +130, which sums to about 103.5%. That extra 3.5% is the overround, and it's how the book makes money no matter which side wins.
The vig (short for vigorish, also called juice or margin) is the overround: the amount by which the two sides' implied probabilities exceed 100%. It costs you because every price you take is shaded against you by that margin. On a standard -110/-110 market the vig is about 4.8%, so you need to win more than half of even-money coin flips just to break even.
Convert both sides to implied probability, add them together, then divide each side by that total. For -150 and +130 the raw figures are 60% and 43.5%, the total is 103.5%, and dividing each by the total gives fair probabilities of about 58.0% and 42.0%. This calculator does that normalisation for you and shows each step.
Yes. It runs in your browser with no login and no signup. Enter one price for a straight conversion, or both sides of a market to see the vig and the fair no-vig probabilities.
More free tools live on the tools hub. If it's fantasy decisions you're working on, the consensus rankings show every player's real finish with the scoring maths visible, the same show-your-work approach this page takes with odds. Want that approach on your whole Sunday? That's powered by gamedai's Scout.