A prediction market price is a probability you can buy. When Josh Allen's NFL MVP contract trades at 27.5¢, the market is saying he has a 27.5% chance of winning the award; when the Democrats trade at 91.5¢ to win the House, the market puts that at 91.5%. No conversion, no formula, no bookmaker's margin hidden in a number. That simplicity is the main thing that makes prediction market odds different from the odds on a sportsbook.
The simplicity only goes so far. Sportsbooks and most betting sites quote American, decimal or fractional odds, so comparing a Kalshi price with a sportsbook line means converting one into the other. Every market also has a bid, an ask and a last price, which can tell three different stories. And fees shift the price you actually pay away from the price on the screen.
This guide covers all of it: how to read a price, how to convert it into every other format, why a market's prices add up to more than 100%, which of the three numbers to trust, and how to compare odds across Polymarket, Kalshi and a sportsbook.
| Price | Implied probability | American odds | Decimal odds | Fractional odds |
|---|---|---|---|---|
| 5¢ | 5% | +1900 | 20.00 | 19/1 |
| 10¢ | 10% | +900 | 10.00 | 9/1 |
| 20¢ | 20% | +400 | 5.00 | 4/1 |
| 25¢ | 25% | +300 | 4.00 | 3/1 |
| 33¢ | 33% | +203 | 3.03 | about 2/1 |
| 40¢ | 40% | +150 | 2.50 | 3/2 |
| 50¢ | 50% | +100 (even) | 2.00 | 1/1 |
| 60¢ | 60% | −150 | 1.67 | 2/3 |
| 67¢ | 67% | −203 | 1.49 | about 1/2 |
| 75¢ | 75% | −300 | 1.33 | 1/3 |
| 80¢ | 80% | −400 | 1.25 | 1/4 |
| 90¢ | 90% | −900 | 1.11 | 1/9 |
| 95¢ | 95% | −1900 | 1.05 | 1/19 |
These conversions assume no margin. Our odds converter handles any price in any direction, and the formulas are below if you want to do it by hand.
Why a Prediction Market Price Is a Probability
Every contract on a prediction market settles at exactly $1 or exactly $0. If the event happens, YES pays $1; if it does not, YES pays nothing. That structure is what turns a price into a probability.
Think about what a fair price would be. If an event has a 30% chance of happening, a contract paying $1 in that case is worth 30¢ on average — you collect $1 three times in ten and nothing the other seven. So a market that is pricing correctly will trade a 30%-likely event at about 30¢. When traders disagree with that price, they buy or sell until it moves, and the price that survives is the market's collective estimate. Our guide to how prediction markets work covers the mechanics of the order book behind this.
Every YES has a NO. On a binary market, buying NO at 70¢ is the same position as selling YES at 30¢, and the two prices should sum to about $1. On a market with many outcomes — a championship, an election field — each outcome has its own YES contract, and all of them together should sum to about $1, since exactly one will win. When they sum to more, that excess is the market's margin, which we cover below.
Converting Prediction Market Odds to American Odds
American odds, the sportsbook standard in the US, express the payout on a $100 stake. Positive numbers show how much a $100 bet wins; negative numbers show how much you must stake to win $100.
The conversion depends on whether the price is below or above 50¢. For a price below 50¢, American odds are +100 × (1 − price) ÷ price. Josh Allen at 27.5¢ is 100 × 0.725 ÷ 0.275, or +264.
For a price above 50¢, American odds are −100 × price ÷ (1 − price). J.D. Vance at 51.2¢ to be the Republican nominee is −100 × 0.512 ÷ 0.488, or −105. Democrats at 91.5¢ to win the House convert to −1076, which is why heavy favourites look so extreme in American format.
To go the other way, from a sportsbook line to a probability: for positive odds, the probability is 100 ÷ (odds + 100), so +300 is 25%. For negative odds, it is the odds ÷ (odds + 100), ignoring the minus sign, so −150 is 60%. Those numbers include the sportsbook's margin, which is the subject of the next section. Live examples of all of these appear in our 2028 Republican nominee odds and NFL MVP odds.
Decimal and Fractional Odds
Decimal odds, standard in Europe and on most crypto betting sites, show the total return per unit staked, including the stake. The conversion from a prediction market price is simple: decimal odds equal 1 ÷ price. The Los Angeles Rams at 11.5¢ to win Super Bowl LXI are 1 ÷ 0.115, or 8.70 — a $10 bet returns $87 in total if they win.
Fractional odds, common in the UK and in horse racing, show profit relative to stake. The conversion is (1 − price) ÷ price, expressed as a fraction. A 20¢ contract is 0.80 ÷ 0.20, or 4/1: win £4 for every £1 staked. Prices that do not reduce to tidy fractions get rounded, which is why fractional odds are the least precise of the three.
Decimal odds have one advantage worth noting: they make comparisons easy. A decimal price of 8.70 on a prediction market against 8.00 at a bookmaker tells you immediately that the prediction market pays about 9% more on the same outcome. Our Super Bowl LXI odds show that comparison for every team.
Why the Prices Add Up to More Than 100%
Add up the price of every outcome in a market and you will usually get a little more than $1. That excess is the overround, and it is the cost of trading built into the prices themselves.
On a sportsbook, the overround is deliberate. A standard line of −110 on both sides of a game implies 52.4% for each team, 104.8% in total. The extra 4.8 points is the book's margin, and the bookmaker's hold works out to about 4.5% of every dollar wagered. On a prediction market, there is no bookmaker, so the overround comes only from the bid–ask spread: to buy every outcome, you pay the ask on each, and the asks sit slightly above fair value.
How big is it in practice? It depends on the market and the venue. Buying one contract on every team at the ask in the Super Bowl market costs about $1.055 on Polymarket and about $1.21 on Kalshi, where many long shots are quoted with a 1–2¢ ask even when they are worth less. For the October Fed decision, the same exercise costs about $1.009 on Polymarket and $1.05 on Kalshi. Our vig guide explains how to measure this for any market, and the vig calculator does it for you.
The overround also tells you where to shop. When one venue's total is much higher than another's for the same market, the gap usually sits in a handful of outcomes — typically the long shots — and those are the outcomes to buy on the cheaper venue. Our roundup of free prediction market tools covers the comparison and calculators that make this quick.
Bid, Ask and Last Price: Which Number Is the Odds
Every market shows at least three numbers, and they can disagree.
The bid is the highest price someone will pay for YES right now — the price you can sell at. The ask is the lowest price someone will sell YES for — the price you can buy at. The last price is where the most recent trade happened. On a busy market, all three sit within a cent. On a quiet one, they can be far apart, and the last price can be stale.
The most reliable single number for "the odds" is usually the midpoint between the bid and the ask. The last price is only as good as the last trade. PredictReport's tracker has recorded clear examples: on September 3, 2026, a Polymarket trade put Dak Prescott's MVP chances at 13.1¢ while Kalshi's midpoint was 6.5¢, and on September 19, a Polymarket trade printed Save the Children at 31.4¢ for the Nobel Peace Prize against a Kalshi midpoint of 12¢. Both prints reversed. A single trade on a thin book is not a consensus, and our guide to prediction market liquidity explains how to tell the difference.
How Fees Change the Odds You Actually Get
The price on screen is not quite the price you pay, because fees add to it.
Kalshi charges 0.07 × contracts × price × (1 − price), rounded up to the next cent. At 40¢, that is about 1.68¢ per contract, so buying 100 contracts at 40¢ actually costs $41.68. Your effective price is 41.68¢, and the break-even probability rises from 40% to about 41.7%. The fee is largest near 50¢ and shrinks toward either extreme. Our Kalshi fees explainer walks through the formula in detail.
Polymarket charges a taker fee of 0.75–1.80% depending on the market category, and nothing for resting limit orders that someone else fills. Using limit orders is therefore the simplest way to make the price on screen the price you pay — our guide to limit orders explains how, and our Polymarket fees explainer covers the category rates. For any combination of price, size and venue, the market fee calculator shows the all-in cost.
Comparing Prediction Market Odds With Sportsbook Lines
To compare fairly, convert both to probabilities and remove the margin from the sportsbook side. Suppose a sportsbook lists a team at −150 and the opponent at +130. The raw implied probabilities are 60% and 43.5%, summing to 103.5%. Divide each by 1.035 and the no-vig probabilities are about 58% and 42%. Now compare those with the prediction market price.
If the prediction market prices the favourite at 55¢, it is offering a better price on the favourite than the sportsbook's fair value suggests; if it prices it at 61¢, the sportsbook is better. This is the core of line shopping, and prediction markets make it more powerful because their margins are thinner. Our comparison of prediction markets and sports betting covers the structural differences, including why exchanges do not limit winning customers.
Reading Odds Across Two Venues
The same event can trade at different prices on Polymarket and Kalshi, because the two run separate order books with different users. On September 21, 2026, J.D. Vance traded at 51.2¢ on Polymarket and 45–46¢ on Kalshi — a six-point gap on the same question.
For a buyer, the rule is simple: buy the side you want wherever it is cheaper. For a trader, a large enough gap can be an arbitrage — buying YES on one venue and NO on the other for less than $1 combined. Most gaps are smaller than the fees needed to trade them, so the useful question is not whether prices differ but whether the difference survives fees. Our arbitrage scanner answers that for every market we track, and Polymarket vs Kalshi explains why the two venues drift apart in the first place.
Common Mistakes When Reading Prediction Market Odds
Confusing the price with the payout. A 20¢ contract does not pay 20¢; it costs 20¢ and pays $1. The payout multiple is 1 ÷ price, or five times the stake.
Ignoring the spread. A market quoted 45¢ bid, 58¢ ask has no meaningful single price. Buying at 58¢ and selling at 45¢ loses thirteen cents immediately.
Trusting a single trade. The last price on a thin book can be one order from one trader. Check the midpoint and the other venue before reacting.
Forgetting time. A contract that settles in two years at 50¢ is not comparable to one that settles next week at 50¢. Money locked for longer has an opportunity cost — Kalshi pays roughly 4% a year on idle cash.
Overrating long shots. Across betting and prediction markets, cheap contracts tend to be overpriced relative to how often they win. Our expected value guide explains how to check whether a price is actually worth paying, and our guide to prediction market accuracy covers the research on this bias.
If you are just starting out, our beginner's walkthrough covers the first trade step by step, the prediction market glossary defines every term used here, and our ranking of the best prediction market apps compares the venues.
Frequently Asked Questions
How do prediction market odds work?
The price of a contract is its implied probability. A contract that pays $1 if an event happens and trades at 30¢ implies a 30% chance. If you buy at 30¢ and the event happens, you receive $1.
How do I convert prediction market prices to American odds?
For prices under 50¢, American odds are +100 × (1 − price) ÷ price; for prices over 50¢, they are −100 × price ÷ (1 − price). A 25¢ contract is +300, and a 75¢ contract is −300. Our odds converter does it instantly.
What does 60 cents mean on Kalshi?
It means the market prices the outcome at about 60%. The contract costs 60¢ plus Kalshi's fee and pays $1 if the outcome happens, which is equivalent to −150 in American odds.
Why don't Polymarket prices add up to 100%?
Because buying every outcome means paying the ask on each, and asks sit slightly above fair value. The excess is the overround. On the Super Bowl market it is about 5.5% on Polymarket and about 21% on Kalshi.
Are prediction market odds better than sportsbook odds?
Usually, because there is no house margin. A two-way sportsbook line typically holds about 4.5%, while a liquid prediction market often holds under 1%. See prediction markets vs sports betting.
Which price should I use — bid, ask or last?
The midpoint between the bid and ask is usually the best estimate of the odds. The ask is what you pay to buy, and the last price can be stale on thin markets.
Do fees change prediction market odds?
Yes. Kalshi's fee adds about 1.68¢ per contract at 40¢, raising your break-even probability from 40% to about 41.7%. Resting limit orders on Polymarket avoid the taker fee.
Why are odds different on Polymarket and Kalshi?
The two venues run separate order books with different users, so prices drift apart. Buy wherever the side you want is cheaper, and use our arbitrage scanner to see when a gap survives fees.
