Implied Probability Calculator
Turn betting odds into a win probability. Enter odds in American (-110, +150), decimal (1.91), or fractional (5/2) format — and optionally the other side of the market to strip out the vig and see the true no-vig probability and the bookmaker’s margin.
Example: with Odds format American (-110, +150) · Odds (your side) -110 · Odds (other side, optional — for vig) -110 → Implied probability: 52.38%.
- Same odds in other formatsDecimal 1.91 · American -110 · Fractional 10/11
- No-vig fair probability50.00% — fair decimal odds 2.00
- Bookmaker marginOverround 4.76% — the bookmaker margin baked into this market
Computed by the calculator below using its default values. Change any input to see your own numbers.
Implied probability = 1 / decimal odds. Both sides of a market imply more than 100% combined — that excess is the vig, the bookmaker’s built-in margin.
From odds to probability — and why the market adds past 100%
Every odds format encodes the same thing: the payout per dollar staked. Decimal odds state total return directly, so implied probability is simply 1 divided by the decimal number — 1.91 implies 52.38%. American odds convert first: +150 returns 2.50 per dollar (40%), while -110 means risking 110 to win 100, a 1.91 return (52.38%).
Here is the catch: at -110 / -110, both sides imply 52.38%, and 52.38 + 52.38 = 104.76%. Real probabilities must total 100%, so the extra 4.76% is the overround — the vig. It means the odds pay less than the true chances justify. Dividing each implied probability by the total recovers the no-vig fair line: 50% each side in this case.
How it’s calculated
All odds convert to decimal first: American positive a → 1 + a/100; American negative a → 1 + 100/|a|; fractional x/y → 1 + x/y. Implied probability = 1 / decimal. With both sides entered, overround = (implied A + implied B) − 100%, and the no-vig fair probability = implied A ÷ (implied A + implied B). Fair decimal odds are 1 / fair probability.
The no-vig method removes the margin proportionally, which is standard but approximate — books sometimes shade favorites and longshots unevenly (favorite-longshot bias).
Common odds and their implied probability
| American | Decimal | Fractional | Implied probability |
|---|---|---|---|
| -200 | 1.50 | 1/2 | 66.67% |
| -110 | 1.91 | 10/11 | 52.38% |
| +100 | 2.00 | 1/1 | 50.00% |
| +150 | 2.50 | 3/2 | 40.00% |
| +300 | 4.00 | 3/1 | 25.00% |
Computed with implied probability = 1 / decimal odds; formats are exact conversions of one another.
Common mistakes
- Reading -110 as a 110% or 11% chance — negative American odds mean risk 110 to win 100, which implies 52.38%.
- Treating implied probability as the true probability: it includes the vig, so a -110 line does not mean the book thinks the side wins 52.38% of the time.
- Forgetting decimal odds include the stake — decimal 2.50 is +150, not +250.
- Adding fractional odds like fractions: 5/2 means 5 profit per 2 staked (implied 28.57%), not five halves of probability.
Frequently asked questions
What is the implied probability formula?
Implied probability = 1 / decimal odds. Convert other formats first: American +150 → decimal 2.50 → 40%; American -110 → 1.91 → 52.38%; fractional 5/2 → 3.50 → 28.57%.
What is the vig and why do both sides add past 100%?
The vig (overround) is the bookmaker margin. At -110 both sides imply 52.38%, totaling 104.76% — the 4.76% excess is what the book keeps on balanced action. Fair odds on a true coin flip would be +100 each.
How do I remove the vig from a line?
Divide each side’s implied probability by the total. For -110 / -110: 52.38 / 104.76 = 50% fair probability each side, or fair decimal odds of 2.00.
Is a bet good if my estimate beats the implied probability?
That is the core idea of value betting: if you believe the true chance is higher than the no-vig implied probability, the bet has positive expected value. The margin for error is small, so compare against the fair (vig-removed) number, not the raw one.