Arbitrage across sportsbooks, Kalshi and Polymarket — measured live, net of every fee
ArbMonster watches 22 venues around the clock, pairs markets that ask the exact same question, walks each order book to a fillable size and subtracts every fee. A card only appears when money is actually left over. Three engines in one dashboard: sportsbook ↔ sportsbook, sportsbook ↔ prediction market, and Kalshi ↔ Polymarket.
How the scanner works
Kalshi lists ~100,000 markets; Polymarket thousands; every sportsbook prices every game. A matcher proposes cross-venue pairs and an automated reviewer confirms they settle identically — the confidence is printed on every card.
Quotes lie about depth. The engine walks each prediction-market order book level by level to your stake and averages the fill; sportsbook depth is unknown, so sizing there is capped conservatively.
Kalshi's 7% × p × (1 − p) taker fee, Polymarket's category rates, and the vig each sportsbook hides in its odds. Only what is left after all of that is called an edge.
What the site publishes
- The measured vig of 20 US sportsbooks, by sport and market type, from live lines — updated every scan, not copied from a press release.
- Every Kalshi↔Polymarket pair we track, with both venues' price history and how often and how long the two disagreed.
- Arbitrage by sport — which books disagree most on MLB, NFL, NBA, NHL and MLS, and what the edges looked like over the last 30 days.
- Calculators for vig, dutching, no-vig odds, odds conversion, and each prediction market's exact fee formula.
- The methodology behind every number, and monthly data reports computed from the archive.
Frequently asked questions
What is ArbMonster?
A market-data service that continuously compares prices for the same outcome across 20+ US sportsbooks, Kalshi and Polymarket, and shows the cases where buying both sides costs less than the payout — net of every venue fee, at a size the order books can actually fill.
What is arbitrage in sports betting and prediction markets?
Backing every outcome of one event across different venues at prices that add up to less than 100% implied probability. The same amount comes back whichever way the event resolves; the profit is the pricing disagreement, not a prediction.
Why are the numbers net of fees?
Because gross gaps are misleading. Kalshi charges 7% × price × (1 − price) per contract on taker fills, Polymarket charges a category rate on the same curve, and sportsbooks hide their cost in the odds. A 3% gross gap is frequently a 1% net edge or nothing at all, so every headline number here already has fees subtracted.
What do the three engines compare?
Sportsbook against sportsbook (identical lines, different prices), sportsbook against prediction market (a moneyline versus a 'team wins' contract), and Kalshi against Polymarket (the same binary question on both exchanges). To our knowledge no other scanner runs all three in one dashboard.
How fresh is the data?
Prediction-market order books stream in live over websockets and cards re-evaluate on every tick. Sportsbook lines refresh every 15 minutes, pre-game only, with a countdown on the dashboard. Any card older than the freshness window is dropped rather than shown stale.
Does ArbMonster place trades or guarantee profit?
No. It never touches a venue account, never places orders, and never promises outcomes. It measures public prices and shows the math; execution risk, venue rules, fees and resolution differences are yours to verify. Nothing on this site is financial or betting advice.
What does the free plan include?
Three live opportunities per day (up to 1% net edge), every tracked market with both venues' price history, the venue-cost tables, calculators and the blog. Core ($39/month) shows every detected opportunity the moment it appears, with venue filters and full line-movement history.