How to read prediction market odds: cents, percent, and lines
Prediction market prices are probabilities. How to convert cents, percent, American, decimal, and fractional odds each way, with formulas, a table, and the vig.
A prediction market price is a probability wearing a different outfit. YES at 34 cents, YES at 34%, and a sportsbook line of +194 all say the same thing: the event happens about a third of the time. The conversions are arithmetic, and the only place the numbers stop matching is the bookmaker's margin.
Cents and percent are the same number
On Kalshi a contract trades between 1 and 99 cents and pays $1. On Polymarket a share trades between $0.01 and $0.99 and pays $1. Either way the price is the fraction of the payout you pay up front, and a fraction of 1 is a percentage. YES at 34 cents is YES at 34%, and we print every price as a percentage for that reason. How yes/no contracts work covers the payout side; What is a prediction market covers why anyone reads the number as a forecast.
Implied probability
The price is called the implied probability because it is the probability at which the contract is fairly priced: paying 34 for a 34% chance at 100 has an expected value of zero. Whether the crowd's 34% is a good estimate is a separate question, covered in Price versus probability. The conversions below all run through implied probability, so convert to a percentage first and then out to whatever format you need.
American odds
The format US sportsbooks use. A negative number is the favorite and says how much you must risk to win 100; a positive number is the underdog and says how much you win on a risk of 100. The favorite at -150 risks 150 to win 100; the underdog at +130 risks 100 to win 130.
To probability:
- Negative odds -X: p = X ÷ (X + 100). At -150, p = 150 ÷ 250 = 60%.
- Positive odds +X: p = 100 ÷ (X + 100). At +130, p = 100 ÷ 230 = 43.5%.
From probability:
- If p is above 50%: odds = -100 × p ÷ (1 - p). At 60%, -100 × 0.6 ÷ 0.4 = -150.
- If p is below 50%: odds = +100 × (1 - p) ÷ p. At 34%, +100 × 0.66 ÷ 0.34 = +194.
- At exactly 50%, both formulas give 100, printed as +100 or -100 (even money).
Decimal odds
The format used in Europe and Australia and by most exchanges outside the US. The number is the total return per unit staked, including the stake. Decimal 2.50 returns 2.50 for every 1.00 risked, a profit of 1.50.
- To probability: p = 1 ÷ decimal. At 2.50, p = 40%.
- From probability: decimal = 1 ÷ p. At 34%, 1 ÷ 0.34 = 2.94.
Decimal odds are the prediction market price turned upside down: a share at 34% has decimal odds of 100 ÷ 34.
Fractional odds
The format of British racing. The fraction is profit over stake: 3/1 ("three to one") wins 3 for every 1 risked; 1/3 ("one to three," or "three to one on") wins 1 for every 3 risked.
- To probability: p = denominator ÷ (numerator + denominator). At 3/1, p = 1 ÷ 4 = 25%. At 1/3, p = 3 ÷ 4 = 75%.
- From probability: fraction = (1 - p) ÷ p, then reduce. At 34%, 0.66 ÷ 0.34 = 1.94, so about 2/1; a bookmaker would quote 15/8 or 2/1.
Conversion table
| Probability | Exchange price | Decimal | Fractional | American |
|---|---|---|---|---|
| 10% | 10 | 10.00 | 9/1 | +900 |
| 20% | 20 | 5.00 | 4/1 | +400 |
| 25% | 25 | 4.00 | 3/1 | +300 |
| 33% | 33 | 3.00 | 2/1 | +200 |
| 40% | 40 | 2.50 | 3/2 | +150 |
| 50% | 50 | 2.00 | 1/1 | +100 |
| 60% | 60 | 1.67 | 2/3 | -150 |
| 67% | 67 | 1.50 | 1/2 | -200 |
| 75% | 75 | 1.33 | 1/3 | -300 |
| 80% | 80 | 1.25 | 1/4 | -400 |
| 90% | 90 | 1.11 | 1/9 | -900 |
Odds converter
A YES at 34% costs 34 and pays 100: profit 66.0 per contract, a return of 194% on cost. Fractional: 1.94 to 1. Implied probability with no vig: 34%.
Exchange prices carry no bookmaker margin, so the price is the implied probability. A sportsbook line converts to a probability that sums above 100% across outcomes; the difference is the vig.
The 33% and 67% rows are rounded; the exact figures are one third and two thirds. Everything else in the table is exact.
The vig: why a sportsbook line and an exchange price disagree
Add up the two sides of a sportsbook line and you get more than 100%. A standard -110 / -110 point spread implies 52.4% on each side, 104.8% in total. The extra 4.8 points is the overround, and the bookmaker's share of the handle, the vig, is that excess divided by the total, about 4.5%. The book has priced both sides so that it profits whichever way the game goes.
Add up YES and NO on an exchange and you get 100%, because one of them will pay 100 and the other 0, and no one is keeping the difference. The exchange's costs are elsewhere: the spread between bid and ask, and a fee on some venues, covered in Payout versus profit. The two costs are not the same size. On a liquid exchange market the spread is a point or two and the fee under 2 cents; on a sportsbook the vig is built into every line at 4 to 5 points on standard markets and more on longshots.
To compare a sportsbook line to an exchange price, strip the vig first: convert both sides to implied probability, then divide each by their sum. That gives the book's fair probability, which is the number an exchange price should be compared against.
A worked example
A sportsbook lists the favorite in the Super Bowl market at -150 and the underdog at +130. On the exchange, the favorite's YES trades at 58% and the underdog's at 42%.
| Step | Favorite | Underdog |
|---|---|---|
| Sportsbook line | -150 | +130 |
| Implied probability | 150 ÷ 250 = 60.0% | 100 ÷ 230 = 43.5% |
| Sum of both sides | 103.5% | |
| Vig-free probability | 60.0 ÷ 103.5 = 58.0% | 43.5 ÷ 103.5 = 42.0% |
| Exchange price | 58% | 42% |
The sportsbook and the exchange agree once the vig is removed. The raw -150 looked like the book was more confident in the favorite than the exchange was; it was not, it was charging 3.5 points of margin and spreading it across both sides. A bettor at -150 needs the favorite to win 60% of the time to break even. A buyer at 58% needs 58%, plus a fee that on Kalshi's schedule as of September 2026 would be 0.07 × 0.58 × 0.42, about 1.7 cents a contract, taking the break-even to about 59.7%. Close to the book, but the exchange position can also be sold before the game, which a bet slip cannot. Sports contracts versus sportsbook bets covers the other differences.
The same arithmetic works on futures. A college football champion line of +600 is 14.3% before vig, and a futures board that adds up to 130% is taking 23 points off the top; an exchange price of 12% on the same team is not lower than the book's real number, it is lower than the book's inflated one. NFL MVP futures are the same story with a longer tail.
What to check before you trade
- Convert everything to a percentage before comparing anything to anything.
- Strip the vig from a sportsbook line by dividing each side by the sum of both.
- Read the exchange bid and ask, not the midpoint, because the midpoint is not a price you can trade.
- Add the fee at that price; on Kalshi it peaks at 1.75 cents a contract near 50%, and Kalshi's own fee page has the current schedule.
- Make sure the two things you are comparing settle on the same rule.
Nothing on this page is investment advice, and the venue's current rule text and fee page are the only things that pay.
Reviewed Thu, Sep 17, 2026. Guides are re-read against the live markets every week; fees, rules, and availability change, and the venue’s own page is the final word.