Arbitrage (betting & prediction markets)
Backing every outcome of the same event across two or more venues at prices that add to less than the payout, so the same amount comes back whichever way it resolves.
Arbitrage exists because venues price independently. A sportsbook, Kalshi and Polymarket each quote their own number for the same outcome, and for short windows those numbers disagree by more than the fees. When the cost of buying both sides is below the fixed payout, the difference is locked in at the moment both orders fill.
The catch is execution: both legs have to fill at the quoted size before either venue moves. Displayed prices overstate what you can actually get, fees quietly eat one to seven percent, and most windows close within minutes. A real arbitrage is one that survives fees, depth and time — which is why ArbMonster shows every opportunity net of all three.
YES costs $0.55 on Kalshi, NO costs $0.42 on Polymarket. Buying both costs $0.97 for a contract that pays exactly $1.00. The $0.03 gap is the gross edge; after each venue's fee it might be $0.015 — still positive, so it is an arbitrage.
Measured on ArbMonster: Live opportunities · How ArbMonster measures it