A prediction market does not quote odds. It sells shares that pay a dollar if an outcome happens, and the price of a share, in cents, is the market's probability. That is simpler than any odds format, but it hides the same things a bookmaker's price hides — margin and the difference between a price and a fair probability — in slightly different places. This guide covers the conversions, the spread, fees, and how to line a Polymarket price up against a sportsbook. If you are new to the platform, How Polymarket works explains the mechanics, the participants, resolution and the legal picture first.
Shares, prices and probability
Every binary market on Polymarket has two shares: YES and NO. Each pays $1.00 if it resolves in its favour and $0 otherwise. If YES trades at 64¢, the market is saying the outcome is 64% likely. Buy 100 YES shares for $64; if YES resolves, receive $100, a profit of $36.
p = price (in dollars) decimal = 1 / p 64¢ → 1.5625 american = (decimal − 1) × 100 if decimal ≥ 2,
else −100 / (decimal − 1) 64¢ → −177.8 → −178 25¢ → 4.00 → +300
The Polymarket converter turns any price into decimal, American and fractional odds and shows the NO side alongside.
The reverse also works: a sportsbook price becomes a Polymarket price by taking its implied probability in cents. -150 is 60% and would trade around 60¢; +200 is 33% and around 33¢.
YES and NO
On a well-traded market the YES and NO prices add up to about $1.00, because holding one YES and one NO share guarantees exactly $1.00 back. If YES is 64¢, NO should be near 36¢. When the sum drifts above $1.00, that excess is the spread.
spread = YES + NO − 1.00 64¢ + 38¢ = 1.02 → 2¢ spreadThe spread plays the role a bookmaker's vig plays: buying both sides costs more than they pay back. A 2¢ spread on a market near 50¢ is comparable to a two-way sportsbook market at about -102 each side, far tighter than the usual -110. Thin markets and longshots have wider spreads; a market at 5¢ / 97¢ carries 2¢ of spread on a 5¢ price, which is proportionally large.
Because you buy at the ask and sell at the bid, the price you see quoted as "the price" is usually the last trade or the midpoint. The price you pay is the best ask. For arbitrage and value calculations use the ask (what it costs to buy), not the midpoint.
Fees
Polymarket's fee structure has changed over time and differs by market category, so this site does not hardcode a rate; the calculators use a placeholder you should replace with the current schedule. Where fees apply they come off the profit, which makes them work like exchange commission on net winnings:
effective = 1 + (decimal − 1) × (1 − fee) 64¢ share, 2% fee: 1 + 0.5625 × 0.98 = 1.551
Gas and withdrawal costs are outside the price and ignored here. On small positions they can matter more than the fee.
Comparing with a sportsbook
Line the two up as decimal odds after fees on the Polymarket side and after nothing on the sportsbook side (its margin is already in the price).
Is 64¢ better than -170?
64¢ is 1.5625 before fees, 1.551 after a 2% fee. -170 is 1.588. The sportsbook is better on this side. On the other side, NO at 38¢ is 2.63 before fees; if the sportsbook offers +150 (2.50) on the opposite outcome, the market is better there. Compare each side separately; the two venues often disagree in opposite directions.
When they disagree enough, the two sides form an arbitrage: buy YES on the market and back the opposite outcome at the sportsbook, or the reverse. The Polymarket vs sportsbook arbitrage guide works through the sizing. The same comparison works between two prediction markets: Kalshi prices contracts in cents the same way, with its own fee schedule, and the two venues often disagree on the same event.
Multi-outcome events
Many events are groups of binary markets: one market per candidate in "who wins the election?", one per team in "who wins the title?". Exactly one resolves YES, so in a consistent group the YES prices sum to about $1.00 and the NO prices to about $(n − 1). When the YES prices sum to less than $1.00, buying every YES in the right proportions locks in a profit; when the NO prices sum to less than $(n − 1), buying every NO does. The group arbitrage calculator checks both baskets and sizes the shares.
Reading a price as probability, honestly
The price is the market's probability with the spread and any fee inside it. To get a fair estimate, use the midpoint between YES and NO on liquid markets, or devig the two sides the way you would a sportsbook market: divide each by their sum. 64¢ / (64¢ + 38¢) = 62.7% for YES. On thin markets the last-traded price can be stale; look at the order book depth before trusting a price on a market with little volume.
Prediction markets also carry a specific kind of risk that sportsbooks mostly do not: resolution. The market resolves according to its written rules, which may differ from the common-sense reading of the question. Read them before comparing the price with anything.
Bids, asks and the price you actually get
A market's headline price hides two numbers. The bid is the most anyone is currently willing to pay for a share; the ask is the least anyone will sell for. Buying takes the ask; selling takes the bid. On a liquid market they sit a cent apart; on a thin one they can be five or ten apart, and the "price" shown between them is a midpoint that nobody is trading at. When you convert a Polymarket price to odds for a comparison with a sportsbook, use the ask if you intend to buy and the bid if you intend to sell, and check how many shares sit at that level. A hundred shares at 64¢ followed by a thousand at 66¢ means a large order fills mostly at 66¢, which is 1.515 decimal rather than 1.5625: a difference of about two points in implied probability.
Time and capital
A sportsbook bet settles when the event ends. A market position can be closed at any time by selling the shares, which makes it more like a trade than a bet: the price moves continuously, and a position that was value at 64¢ can be sold at 70¢ before resolution for a profit that does not depend on the outcome. The flip side is capital lockup on long-dated markets. A share at 90¢ on an event nine months away pays 11% if it resolves YES, which is about 14% annualised before fees; whether that beats other uses of the capital is a question a sportsbook bettor never has to ask. Compare a market price with a sportsbook price only when the two settle on the same timescale, or discount the market's return for the wait.
Resolution rules
Every market resolves according to its written rules, and the rules are the market. A question that reads "Will X happen by June?" may resolve on a specific source's announcement, on a UTC deadline, or on an interpretation the rules spell out; edge cases (postponements, disputed results, a source that goes silent) are decided by the resolution process, not by common sense. Before converting a market price to odds and comparing it with a sportsbook, read the rules and confirm that the two venues would settle the same way on every plausible ending. A price that looks like value against a sportsbook line is often just a different question.
Scale and slippage
Sportsbook prices are fixed for the stake they accept; market prices depend on how much you buy. A small order fills at the best ask; a large one walks up the book, paying more per share as each level is exhausted. The average price of a large order is what matters for conversion, and it can differ from the displayed price by several cents on a thin market. Before treating a market price as an odds quote, look at the depth: the number of shares available within a cent or two of the best ask. If your intended size exceeds it, price the order at the level where it would finish, not where it starts.
Selling before resolution has the same problem in reverse. A position that shows a paper profit at the midpoint may realise much less if the bid side is thin. This is the main practical difference from a sportsbook, where the price you took is the price you get, and it argues for smaller positions in less liquid markets whatever the apparent edge.
Cents, probabilities and rounding
Because prices are quoted in whole cents, the finest distinction a market can make is one percentage point, which is coarser than most sportsbook price ladders near even money (a tick from -110 to -108 is about half a point). Near 50¢ this hardly matters; near 5¢ or 95¢ a single cent is a large proportional move, and the equivalent odds jump accordingly: 5¢ is 20.0 decimal, 6¢ is 16.7. When comparing longshots across venues, remember that a market cannot express the price in between.
Common mistakes
- Reading the last trade as the probability. On a thin market the last trade can be hours old and several cents from the current bid and ask. The midpoint of a liquid book is the best single estimate; the ask is what you will pay.
- Converting the midpoint and then buying at the ask. The odds you computed are not the odds you got. Convert the ask if you are buying and the bid if you are selling.
- Forgetting the fee. A 64¢ share is 1.5625 only before fees. Apply the fee on profit before comparing with a sportsbook, whose margin is already in its price.
- Comparing the wrong side. A sportsbook -170 is on the outcome; the market's NO at 38¢ is on its opposite. Line up YES with the sportsbook's price on the same outcome and NO with the price on the other, and check each side separately.
- Ignoring resolution rules. Two venues can settle the same question differently. A gap that survives fees and spread is sometimes a different question, not an arbitrage.
- Sizing on displayed depth. The share count the group arbitrage calculator reports assumes the whole order fills at the quoted price. Check the depth at that level first.
Quick reference
| Price | Probability | Decimal | American |
|---|---|---|---|
| 10¢ | 10% | 10.00 | +900 |
| 25¢ | 25% | 4.00 | +300 |
| 40¢ | 40% | 2.50 | +150 |
| 50¢ | 50% | 2.00 | +100 |
| 64¢ | 64% | 1.5625 | -178 |
| 75¢ | 75% | 1.33 | -300 |
| 90¢ | 90% | 1.11 | -900 |
Every cent value from 1¢ to 99¢ has its own page under Polymarket prices with payouts, fee-adjusted odds and neighbouring prices.