Polymarket, Kalshi and Bookmaker Arbitrage: How It Works
A $0.45 contract means odds of 2.22. Why exchanges diverge from bookmakers and what Dozor measured: 1,129 surebets with Polymarket, 187 with Kalshi in 20 hours.
A surebet between a prediction market and a bookmaker means buying a contract on one outcome on the exchange and betting on the opposite outcome with a bookmaker, when the contract price and the odds together give a sum of inverse odds below one. Over 20 hours on September 5–6, 2026, the Dozor scanner found 1,129 such surebets involving Polymarket and 187 involving Kalshi — one in every nine surebets that day. Their median lifetime was 4.1 seconds, twice as long as average.
How a contract price becomes odds
On a prediction market, an outcome trades as a contract that pays $1 if the event happens and $0 if it doesn't. The contract price is the probability in the market's eyes, and the equivalent odds equal one divided by the price.
| Contract price | Odds | What it means |
|---|---|---|
| $0.30 | 3.33 | the market gives the outcome 30% |
| $0.45 | 2.22 | 45% |
| $0.50 | 2.00 | even chances |
| $0.70 | 1.43 | 70% |
From there, the math is the usual one. A “Team A wins” contract at 0.45 (odds of 2.22) and a bet on Team B with a bookmaker at 2.05: 1/2.22 + 1/2.05 = 0.450 + 0.488 = 0.938. The sum is below one; ROI is 6.6% before fees.
Where the discrepancies come from
A bookmaker moves its line with an algorithm and a trader after every event in the match. An exchange moves its price only through a trade: until someone places a new order, the old price stands. In a live CS2 match, a round ends, the bookmaker updates its odds within a second, and on the exchange the price changes whenever one of the participants decides to trade. Those seconds are the surebet.
The second reason is a different set of participants. Exchange prices are set by traders with their own view of the match, which often differs from a bookmaker analyst's assessment. In tier-2 esports these discrepancies hold longer.
What the measurement showed
Data from Dozor's surebet history from 21:54 UTC on September 5 to 17:48 UTC on September 6, 2026: 10,000 records, 27 books.
| Metric | Polymarket | All surebets |
|---|---|---|
| Surebets in the window | 1,129 | 10,000 |
| Median lifetime | 4.1 s | 2.1 s |
| Most frequent pairs | Duel 145, Winline 95, Pinnacle 94, Marathon 94 | Betcity–Duel 314 |
Kalshi was involved in 187 surebets. Among the 483 surebets that lasted longer than a minute, the Polymarket–Winline (21) and Duel–Polymarket (21) pairs are in the top five, alongside CSGOEmpire–Melbet, 1win–BC.Game and Duel–Thunderpick.
What goes into the calculation besides odds
- Exchange fee. Polymarket charges a fee on profit in some markets; Kalshi charges a fee per trade. It is deducted from the payout on the exchange leg and lowers ROI by a fraction of a percent.
- Spread. In the order book, the buy price is higher than the sell price. Odds for a surebet are calculated from the price at which you can actually buy the contract, not from the middle of the spread.
- Order book depth. At 0.45 there may be only $200 worth of contracts. A stake the calculator worked out for a 100,000 ₽ bankroll won't fit: part of it will fill at a higher price. Dozor shows depth and filters surebets by minimum volume.
- Currency. The exchange leg is in dollars or USDC, the bookmaker leg in rubles. The exchange rate at the time of the trades goes into the calculation, and currency swings before the match is settled are a separate, small risk.
- Contract rules. A contract is defined in text: what counts as a win, and what happens if the match is canceled or postponed. The bookmaker's rules for the same match may differ. Open both descriptions before placing a surebet.
Why this beats a surebet between two bookmakers
An exchange doesn't limit accounts: it earns on fees and has no interest in the customer losing. An exchange account won't get restricted for systematic play, the way a bookmaker account does within weeks. There's only one constraint, and it's an honest one: order book volume. For an arber who already has half their bookmaker accounts limited, the exchange becomes a second leg that isn't going anywhere.
The flip side is KYC and jurisdiction. Kalshi is available to US residents; Polymarket restricts a number of countries. Check availability before building a strategy on an exchange.
How it looks in Dozor
Polymarket and Kalshi are connected as regular books. Their contracts are mapped to the same markets as bookmaker lines: match winner, map winner, and totals wherever the exchange trades them. In the card, the exchange leg shows the contract price, the equivalent odds and the order book depth. The “minimum exchange order book depth” filter drops surebets you can't get into with the stake you need. More on the page about surebets with prediction markets.
FAQ
Can I treat the contract price as odds directly?
Yes, the odds equal 1 / price. Account for fees and the spread separately.
Are there surebets between Polymarket and Kalshi?
On major events that both platforms trade, yes. In esports, Kalshi's volume is small, and most surebets run against bookmakers. Live exchange numbers are on the Prediction markets page.
Why do exchange surebets last longer?
The price changes through a trade, not an algorithm. A median of 4.1 seconds versus 2.1 in the measurement.
Can I sell a contract before the match ends?
Yes, that's how an exchange differs from a bet. If the second leg at the bookmaker didn't go through, you can close the contract at the current price instead of waiting for the result.