Convert decimal, American, fractional odds or a Polymarket cent price into the probability the market is actually charging you for.
Decimal 2.50 implies a 40% chance — the fair probability this price demands before a bet is break-even.
Add the opposite side's odds to see the market's built-in margin.
Every price in a betting or prediction market is a probability in disguise. When a bookmaker or a Polymarket order book quotes a number, that number already encodes how likely the outcome is assumed to be. Stripping the price back to that underlying chance is what this calculator does.
On a prediction market the relationship is unusually direct. A contract that pays $1 if an event happens and $0 if it does not will trade at a price equal to the crowd's probability estimate. A YES share at 62 cents is the market saying "62% likely." There is no vig hidden inside a single price the way there is in traditional sports betting — instead the cost shows up in the spread between the buy and sell side, and occasionally in the fact that YES plus NO can exceed 100%.
For anyone sizing a position, implied probability is the reference number. It is the bar your own research has to clear. Everything else in prediction market strategy — expected value, Kelly sizing, bankroll management — is built on top of it.
All four conventions describe the same underlying thing. Decimal odds are the simplest to work with mathematically because the inverse is a single division, which is why quantitative traders tend to convert everything to decimal before doing anything else. American odds split into two branches depending on sign, which is the most common source of hand-calculation errors.
| Decimal | American | Fractional | Implied probability |
|---|---|---|---|
| 1.25 | −400 | 1/4 | 80.0% |
| 1.50 | −200 | 1/2 | 66.7% |
| 2.00 | +100 | 1/1 | 50.0% |
| 2.50 | +150 | 3/2 | 40.0% |
| 4.00 | +300 | 3/1 | 25.0% |
| 10.00 | +900 | 9/1 | 10.0% |
Notice the asymmetry that trips people up. A 50% probability corresponds to decimal odds of exactly 2.00, so any decimal price below 2.00 implies a better-than-even chance. Longshots compress: a jump from 10% to 5% probability moves decimal odds from 10.00 to 20.00 — a dramatic-looking price change for a modest shift in belief.
A fair book would price both sides so their implied probabilities sum to exactly 100%. Real markets rarely do. If YES is offered at implied 55% and NO at implied 48%, the two add to 103%. That extra 3% is the margin — the operator's revenue, and the bettor's structural disadvantage.
On Polymarket the situation is different and often favourable. Because YES and NO are two sides of the same order book, their prices typically sum to very close to 100%. When they sum to slightly less than 100% — say YES at 48¢ and NO at 49¢, totalling 97¢ — buying both sides returns $1 for $0.97, a risk-free profit if you can fill both legs. That discrepancy is the raw material of prediction market arbitrage, and it is exactly why measuring the sum matters.
The chance of an outcome that a quoted price already assumes, found by inverting that price.
No. Polymarket cents are already probabilities. Divide by 100 only to get a decimal for use in a formula.
Traditional sportsbooks commonly build in 2–7%. Polymarket's order book often sits near 100%, sometimes below it.
Implied probability is the input that makes the EV calculation possible. Without it there is no baseline to compare your estimate against.
Once you know the implied probability and your own estimate, the Kelly Criterion Calculator tells you exactly how much of your bankroll the edge justifies.