Turn any betting odds into the win probability the market is charging. Enter American, Decimal, or Fractional odds for an instant implied probability — with the exact formula shown — or switch to No-Vig mode to strip a two-way market down to each side's fair probability and the sportsbook's hold.
Implied probability is the win probability baked into a price. When a sportsbook posts a number, it is really quoting a probability — the chance the outcome happens, as the book has priced it — and the odds are just that probability dressed up in betting notation. Reading the implied probability back out is the single most useful thing you can do with a line, because it puts the market on the same scale as your own opinion. If you think a team wins 55% of the time and the market implies 48%, you have found a number you can act on. If the market implies 60%, you do not.
Every odds format encodes the same probability; they just look different. Decimal is the cleanest to reason about: implied probability is simply one divided by the decimal price. The American and fractional formulas below get you to the same place from the U.S. and UK conventions.
| Format | Implied Probability Formula | Example |
|---|---|---|
| Decimal (d) | P = 1 / d | 2.50 → 1 / 2.50 = 40.00% |
| American − (favorite) | P = (−odds) / (−odds + 100) | -150 → 150 / 250 = 60.00% |
| American + (underdog) | P = 100 / (odds + 100) | +130 → 100 / 230 = 43.48% |
| Fractional (a/b) | P = b / (a + b) | 5/2 → 2 / 7 = 28.57% |
A quick sanity check: shorter prices (bigger favorites) produce higher implied probabilities, and longer prices (bigger underdogs) produce lower ones. Decimal 2.50, American +150, and fractional 3/2 are the same bet — all imply 40%.
Say you are looking at a -150 favorite. Plug it into the American-negative formula: 150 / (150 + 100) = 150 / 250 = 0.60, or 60%. The book is charging you as if this side wins 60% of the time. In decimal that price is 1.667, and 1 / 1.667 = 0.60 — same answer, different notation. Now the only question that matters is whether your honest estimate of this team's chances is above 60%. If it is, the bet has positive expected value before vig; if it is not, you are paying the book for the privilege of a likely-losing wager.
Here is the catch with raw implied probability: the two sides of a market never sum to 100%. A -110 / -110 game implies 52.38% + 52.38% = 104.76%. That extra 4.76% is the sportsbook's hold — the margin built into the price. So the raw 52.38% overstates each side's true chance.
To get the market's best estimate of the true probability, you remove the vig: take each side's raw implied probability and divide it by the sum of both. For -110 / -110 that is 52.38% / 104.76% = 50% on each side, with a hold of about 4.55%. The No-Vig tab above does this automatically for any two-way market. De-vigged probabilities, not raw prices, are what you should compare your model against — and they are the foundation of closing line value math. For the fair odds rather than the fair probability, use the No-Vig Fair Odds Calculator; to convert any single price across all four formats at once, use the Odds Converter.