Reading Kalshi and Polymarket Prop Prices
Prediction markets are the odd ones out on a prop board. Sportsbooks post a line and two American-odds prices. Kalshi and Polymarket show prices for contracts that traders buy and sell among themselves. This article covers what those prices mean and what to watch when you set one beside a sportsbook line. For the arithmetic of converting between formats, see prediction market prices vs sportsbook odds. This piece is about reading them on an actual prop.
A contract is a yes/no question
A prediction-market prop is usually phrased as a question, such as "Will this player score 25 or more points?" A Yes contract pays 1 dollar if the answer is yes and nothing if not. A price of 55 cents means the market's implied chance is about 55%. The glossary defines prediction markets and implied probability in a line each.
Two details matter for props.
The threshold is a whole number, the book's line often is not. A question that reads "25 or more points" is the same event as a sportsbook's "over 24.5," since points come in whole numbers. Match the two by thinking through what actually wins, not by comparing the digits.
Several thresholds can exist for one player. A prediction market may list 15+, 20+, 25+ and 30+ as separate contracts, while a sportsbook shows one main line plus its alt lines.
Each side has its own price
Yes and No are separate contracts, and their buy prices need not add up to exactly 100 cents. ArchivePicks shows each side at the price someone could actually buy it at, rather than assuming one side is simply 100 minus the other. See the methodology.
That matters because the gap between what buyers pay and sellers ask is a real cost, and it is easy to hide by quoting a midpoint.
A worked example (hypothetical numbers)
These numbers are invented.
A sportsbook posts an NBA rebounds prop at 9.5 with the over at -120 and the under at +100. A prediction market lists "10 or more rebounds" with Yes at 56 cents and No at 46 cents.
- The over at -120 implies about 54.5% (120 / 220).
- The under at +100 implies 50%. The book's two sides total about 104.5%, which is its margin.
- The market's Yes at 56 cents implies 56%, and No at 46 cents implies 46%. Together they total 102%, so the two buy prices leave a small gap.
The book's margin is bigger than the market's gap, so compare after removing it. The book's fair over chance is about 52% (54.5 divided by 104.5). The market's 56% Yes is a few points higher. In this made-up case that difference could be timing, the market's spread, fees, or a real disagreement, and the numbers alone cannot say which. The example does not show that either source is right.
What to watch
- Liquidity. A thinly traded contract can have a wide gap between the Yes buy price and the No buy price, and a displayed price may not be one you could fill in size.
- Fees. Venues charge in different ways. A price is not the same as your net cost; check the venue's fee schedule.
- Timing. Prediction-market prices move with every trade. ArchivePicks refreshes lines every couple of minutes, so what you see can already have moved.
- Settlement rules. How a contract settles if a player is inactive, or which source decides the stat, can differ from a sportsbook. Read the contract rules.
- Availability. What is legal and open to you depends on where you live, and a stats site cannot tell you that.
How ArchivePicks uses them
For NFL and NBA props, Kalshi and Polymarket are two of five sources, alongside Courtside, Underdog and Novig. Their prices are converted to American odds and shown in the same sportsbook panel so you can read across. Where three or more sources post the same number they also feed the +EV tag. To see how the books compare in general, read comparing a prop across books. ArchivePicks does not trade, hold money or tell you which side of a contract to buy.
FAQ
Is a prediction-market price more accurate than a sportsbook line? There is no general answer, and ArchivePicks does not rank sources. Each has its own structure, costs and traders.
Why is there no prediction-market price on some props? A contract only exists if the venue lists that player, stat and threshold. Not every prop has one.
Do Yes and No always add up to 100 cents? No. The two buy prices can add up to a little more or less. ArchivePicks shows each at its own buy price.
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