Understanding Polymarket Odds: How Prices Become Probabilities

A Polymarket price is a probability with a dollar sign in front of it. Every share is priced between $0.00 and $1.00 and a winning share redeems for $1.00, so Yes at 65 cents means the market puts that outcome at roughly a 65 percent chance. Read the price as a percentage and the rest is arithmetic.

Every price is a probability, and the payout is always a dollar

As of September 2026, Polymarket's documentation states that every share is priced between $0.00 and $1.00, that $1 of collateral creates one Yes share and one No share, and that winning shares redeem for $1.00 while losing shares are worth $0.00. The payout is fixed, so the price carries all the information: "The price represents the market's belief in the probability of that outcome occurring." Polymarket US shows the same idea twice over, a percentage beside a cent price for the same outcome.

Buy Yes at $0.65 and you risk 65 cents to make 35 cents, so you break even only if events priced that way happen about 65 percent of the time. Polymarket is not the house; it states "All trades happen peer-to-peer." If the mechanics are new, read what Polymarket is and browse live markets.

Yes plus No is about a dollar, and the gap is your cost

Polymarket's order-book documentation describes a Yes limit order at $0.60 matching a No order at $0.40: "Since $0.60 + $0.40 = $1.00, the orders match," creating one share of each.

The card price is not necessarily tradeable. It is the midpoint of the bid-ask spread, the gap between the best resting buy and sell orders, so a $0.34 bid and a $0.40 ask display as $0.37. Above a $0.10 spread, Polymarket shows the last traded price instead.

You buy at the ask and sell at the bid, so round-tripping through a 6-cent spread costs 6 cents before fees.

The best ask on Yes plus the best ask on No normally sums to a bit more than $1.00; that excess is the spread, and it is the round-trip cost of trading.

Only takers pay fees; Polymarket states "Makers are never charged fees." The fee is shares times a category rate times the price p times (1 minus p), so it peaks when the price is 50 cents and shrinks symmetrically toward both extremes: a trade at 30 cents costs the same as one at 70 cents. See our fees explainer.

Converting prices to American, decimal and fractional odds

Let p be the price in dollars, so 65 cents is p = 0.65.

Decimal odds = 1 divided by p. At 65 cents: 1.54.

American odds for a favorite, p above 0.50 = minus (p divided by (1 - p)) times 100. At 65 cents: -186.

American odds for an underdog, p below 0.50 = ((1 - p) divided by p) times 100. At 20 cents: +400.

Fractional odds = (1 - p) divided by p. At 20 cents: 4/1. At 65 cents: 7/13.

Reverse it to check a sportsbook line: decimal odds d imply 1 divided by d, +250 implies 100 divided by 350 or 28.6 percent, and -150 implies 150 divided by 250 or 60 percent.

Polymarket's displayed midpoints sum to $1.00, with your cost visible in the spread and the taker fee.

What moves a price, and whether you can act on it

Polymarket states that prices are "a function of realtime supply & demand." A price moves when someone crosses the spread by taking a resting order, or when resting orders are pulled.

Two reported fields tell you whether a number means anything: liquidity ("Reported market liquidity") and volume24hr ("Trading volume over the past 24 hours"), published with bestBid, bestAsk, spread and oneDayPriceChange.

Volume is money that actually changed hands, so treat it as a confidence weight. A 12 percent probability on almost no volume can be one person's opinion.

Polymarket notes the book has "no trading size limits" but warns "large orders may move the price significantly," so check depth first. A thin market can print an extreme price on a small order and sit there for hours. That is an absence of information, not information.

Filter on the screener and watch what repriced today on 24h movers.

Where prices can be wrong

Longshot pricing. Betting markets carry a long-documented favorite-longshot bias, where "on average, betters tend to overvalue 'longshots' and relatively undervalue favorites"; in horse racing that means long-run losses near 5 percent on favorites against roughly 40 percent on longshots. Whether Polymarket tilts the same way is worth testing rather than assuming.

Time value. A share bought at 98 cents that settles nine months out returns at most 2 cents on 98 cents of locked capital, roughly 2 percent before fees, and an adverse resolution still costs the full 98 cents. Check settlement dates on the resolution calendar.

Resolution ambiguity. Polymarket warns that the resolution rules, not the market title, define how a market resolves. Resolution runs through the UMA Optimistic Oracle: a proposer posts an outcome with a bond, typically $750; a 2-hour challenge period follows; a dispute goes to a UMA token holder vote of about 48 hours, taking roughly 4 to 6 days end to end. A rare "Unknown/50-50" case redeems each token for $0.50. A 95-cent price is a view on the event, not on how the rules will be read.

Reading our odds pages and market pages

2026 midterm odds tracks live prices on control of Congress, so you watch the implied probability move, not one snapshot.

Fed rate cut odds splits cut, hold and hike per FOMC meeting, the clearest case of a price doing what a poll cannot.

All markets and the screener let you sort by volume before you trust a price.

24h movers shows what repriced today, usually where new information landed.

The whale tracker and free alerts feed show the size behind a move.

Then open the market itself and read its resolution rules before sizing anything.

Where you can trade, and the rules that apply

The main platform is not available to US residents for trading; from outside the United States you can open Polymarket, subject to the rules of your jurisdiction. US traders use Polymarket US, run by QCX LLC, which the CFTC designated as a contract market in July 2025, and availability varies by state; our Polymarket US guide has the detail. The free tools above work wherever you are.

Circumventing a geographic restriction violates the terms, and we do not cover how.

Frequently asked questions

Are Polymarket odds accurate?

Polymarket publishes its own scorecard: as of September 2026 it reports 90.0 percent accuracy one month before resolution and 98.5 percent four hours out, with a Brier score of 0.0627. Read the definition, because accuracy there is how often "the final leading outcome matched reality," which measures direction rather than calibration: it does not test whether 70-cent markets resolve Yes about 70 percent of the time. The Brier score, which averages squared prediction error where 0 is perfect, is the more informative figure. The same page reports 26.2 percent of resolved markets resolving Yes and 73.8 percent resolving No, so cheap Yes shares are usually cheap for a reason.

Why don't Yes and No add up to exactly 100 percent?

By construction they should: $1 of collateral creates one Yes and one No share, and matching limit orders sum to $1.00. But you are looking at midpoints from two sides of an order book, and the best ask on Yes plus the best ask on No usually sums to slightly more than a dollar. That excess is the spread, the round-trip cost of trading, not a bookmaker's margin.

What does volume mean on a Polymarket market?

Polymarket reports volume24hr as trading volume over the past 24 hours, separately from a reported liquidity figure. Use it as a confidence weight: heavy volume means many people risked money at that price, while near-zero volume can mean a single resting order. Sort by volume on the screener before acting on an odd-looking number.

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