Implied Probability Calculator: Convert Any Odds Format to Win Percentage
Implied probability is the win percentage embedded in a posted price, including the bookmaker's margin. At -200 the implied probability is 66.7%, but the true win probability is lower once you strip the vig. This calculator converts between American, decimal, fractional, and probability simultaneously, and includes bet sizing so you can see actual dollar payouts alongside every format.
Why implied probability includes vig and what to do about it
At -200, the implied probability is 1 / (200/100 + 1) = 66.7%. In a two-sided market at -200/+170, the two implied probabilities sum to more than 100% - the excess is the vig. The book embeds margin on both sides so they profit regardless of outcome.
To find the fair probability, use the no-vig calculator on both sides. Strip the margin and the probabilities sum to 100%. For a -200/+170 market, fair probability works out to roughly 64.4% for the favorite rather than 66.7% - a meaningful difference when evaluating edge.
When you are shopping for the best price, a higher implied probability on the side you want to fade means a lower implied probability on your side, which means better value. Reading probability directly helps you compare formats across books quickly.
How the four formats relate to each other
American odds use +/- notation around 100. Positive odds show profit on a $100 stake; negative show the stake needed to win $100. Decimal odds include the returned stake - 2.00 decimal doubles your money. Fractional odds express profit relative to stake: 3/1 means $3 profit per $1 staked.
All three formats convert through the same probability backbone. Decimal 2.00 = American +100 = Fractional 1/1 = 50% implied probability. Decimal 1.50 = American -200 = Fractional 1/2 = 66.7%. No precision is lost converting between them.
Bet Amount scales the To Win and Payout readouts but does not affect implied probability or odds conversions. Set it to your planned stake so the payout numbers are meaningful.
Step by step: using this calculator
Click into whichever field matches how your source quotes the price. The other fields update as soon as the input parses successfully. The active field is the one driving the calculation.
Switching from one active field to another is straightforward: click the new field and type. The previous field retains its last parsed value but is no longer driving the calculation.
- Set Bet Amount to your planned stake.
- Click into Decimal, American, Fraction, or Probability % - whichever matches your source.
- Type the odds.
- Read all converted formats plus To Win, Payout, and Implied Probability.
Worked example: -200 American and the vig it hides
You see a moneyline favorite at -200. Decimal: 1.50. Fractional: 1/2. Implied probability: 66.7%. On a $100 stake, To Win is $50, Payout is $150.
Now look at the opposing side at +170. Implied probability: 37.0%. Sum: 66.7% + 37.0% = 103.7%. Hold is 3.7%, which is reasonable for a sharp book.
True probability after stripping vig: 66.7 / 103.7 = 64.3% for the favorite. If your model says the favorite wins 60% of the time, you are looking at a -EV bet even though the market is pricing it as a heavy favorite.
Decimal
2.50
Fractional
3/2
Implied probability
40.0%
Common mistakes reading implied probability
Treating implied probability as the true win probability is the fundamental error. It always includes margin. Use the no-vig tool to find fair probability.
Mixing active fields mid-edit causes stale values in inactive fields. When you switch to a new active field, the previous one holds its last parsed value until you clear and re-enter it.
Expecting Bet Amount to change implied probability - it only scales payout math. Probability is determined entirely by the odds.
What most calculator pages skip
This calculator defaults to your local odds format automatically - US visitors see American, UK visitors see fractional, Australian visitors see decimal. No settings to change. Enter the number as it appears on your screen and the rest fills in immediately.