How Polymarket Odds Work: Prices as Probabilities

Polymarket odds look unfamiliar if you are used to bookmakers, but the idea is simple. Each outcome is a share that pays $1 if it is true and $0 if it is not. What people are willing to pay for that share tells you what they think the odds are.

This guide turns prices into probabilities, decimal odds and payouts, then covers the spread, why prices move and how to read a price chart. Facts about how Polymarket works come from its documentation.

Price equals probability

Polymarket's docs say every share is priced between $0.00 and $1.00, and "the price directly represents the market's belief in the probability of that outcome." Their own examples: $0.25 is a 25% chance, $0.50 is 50% and $0.75 is 75%.

Winning shares redeem for $1.00 and losing shares become worthless, per the resolution rules. So a share priced at 65¢ is a bet that the true chance is above 65%.

Prices come from supply and demand. Polymarket does not set them. Traders place orders into a shared order book and trade with each other rather than against a house (Polymarket 101).

From price to decimal odds and payout

To convert, divide 1 by the price. Here is the arithmetic for a $100 stake, ignoring fees:

Price	Implied probability	Decimal odds	American odds	Profit if you win
10¢	10%	10.00	+900	$900.00
25¢	25%	4.00	+300	$300.00
40¢	40%	2.50	+150	$150.00
50¢	50%	2.00	+100	$100.00
75¢	75%	1.33	−300	$33.33
90¢	90%	1.11	−900	$11.11

The formulas are ours, not Polymarket's: decimal odds = 1 ÷ price. For American odds, a price under 50¢ gives +100 × (1 − p) ÷ p, and a price over 50¢ gives −100 × p ÷ (1 − p).

A worked example

Say a market asks whether a team wins and Yes trades at 35¢. You buy 200 shares.

• Cost: 200 × $0.35 = $70.
• If Yes wins: the shares pay 200 × $1 = $200, so your profit is $130. That is decimal odds of 2.86 (1 ÷ 0.35).
• If No wins: your shares are worth $0 and you lose the $70.

Your break-even is the price itself. Buying at 35¢ only makes sense if you think the true chance is above 35%, and a bit above once you add fees.

You do not have to hold to the end. Polymarket's 101 page notes you can sell before resolution to lock in a profit or cut a loss. If the 35¢ shares rise to 50¢, selling 200 of them returns $100, a $30 gain, whatever the final outcome (before fees and spread).

Yes, No and multi-outcome markets

Every market has a Yes and a No side. In the docs' price-discovery example, a Yes buyer at $0.60 and a No buyer at $0.40 are matched because the two prices add to $1.00.

Events can contain several markets. In a multi-outcome event where only one outcome can win, the Yes prices across all outcomes should add up to roughly 100%. When they do not quite, that is usually the spread and thin trading, not free money.

Browse live examples on our events page and compare how prices spread across candidates or scores.

The spread: the price you see is not the price you pay

Polymarket shows the midpoint of the best bid and best ask. If the highest buyer offers 34¢ and the lowest seller asks 40¢, the display shows 37¢. The docs add that if the spread is wider than $0.10, the last traded price is shown instead.

You will not trade at 37¢. A market order pays the ask (40¢) when buying and receives the bid (34¢) when selling.

Here is what that costs on 100 shares, using the docs' example prices:

• Buy at the ask: 100 × $0.40 = $40.
• Sell straight back at the bid: 100 × $0.34 = $34.
• Round-trip cost: $6, or 15% of the stake.

Tight spreads mean liquid markets. On popular events the gap can be a cent or less, but on obscure markets it can dwarf any fee. A limit order lets you name your price and wait, at the risk of not being filled.

Polymarket also notes that its order book has no size limits, but large orders "may move the price significantly," so check order-book depth before trading in size.

Why odds move

Prices change whenever traders disagree with the current one. The common causes:

• New information. A poll, an injury, a court ruling or a data release makes traders re-price.
• Large orders. A big buy takes the cheapest asks first and pushes the price up. Our whale feed shows $10,000+ trades as they land.
• Thin books. With few resting orders, a small trade can swing a price a long way.
• Time passing. As an end date nears with nothing happening, prices for unlikely events tend to drift toward $0, and likely events toward $1.
• Resolution risk. Markets resolve by their written rules, not the headline. Polymarket tells traders to read them, because "the market title describes the question, but the rules define how it resolves."

A big move is not proof that someone knows something. Our guide to reading whale trades covers telling signal from noise.

How to read a price chart

A Polymarket chart plots the market's price (its implied probability) over time. A few habits make it more useful:

1. Check the time range. A 1-day spike and a 1-month trend say different things.
2. Look at volume, not just the line. A jump on thin volume is weaker evidence than one on heavy trading.
3. Find the news. Match the move to what happened. If you cannot, treat it with caution.
4. Watch the extremes. Above 95% or below 5%, small price changes mean large changes in odds. The difference between 95% and 98% is odds of 1.05 versus 1.02, but the chance of an upset dropping from 5% to 2% is a 60% fall.
5. Remember it is a consensus. The line shows what traders will pay, not what is true.

Comparing your own view with the price

The price only matters next to your own estimate. If you think the true chance is q and a share costs p, your expected profit per share before fees is q − p (our arithmetic).

Say you believe an outcome has a 45% chance and Yes costs 35¢. Over many similar bets you would expect to gain about 10¢ per share, so 200 shares would be worth roughly $20 in expectation. If your estimate is 30%, the same share has a negative expectation and the right move is to skip it.

Two cautions. Your estimate is itself a guess, so a small gap between q and p is not much of an edge once the spread and fees are paid. And an expectation is an average: any single bet still ends at $0 or $1 per share.

Common mistakes when reading odds

• Treating a high price as certainty. A 90% share loses one time in ten.
• Ignoring the spread. The displayed midpoint is not your fill price.
• Mixing up Yes and No. Buying No at 30¢ is a bet that Yes is below 70%.
• Trusting thin markets. A price set by a few small orders is a weak estimate.
• Skipping the rules. The title is a summary, and the written rules decide the result.

Odds are not guarantees

A 90% price does not mean "sure thing." It means traders collectively price a 1-in-10 chance of failure, and you can lose your whole stake on those. A price is also only as good as its resolution rules and the traders behind it.

Never stake money you cannot afford to lose. If you are ready to try it, our step-by-step first trade guide walks through the account, the deposit and your first order, and availability by country tells you whether Polymarket is open where you live.