When two books disagree enough, backing every outcome locks in a profit. Check the math and split the stake exactly.
Convert every leg to an implied probability (1/decimal). If the probabilities sum to less than 100%, an arbitrage exists. Stake each leg in proportion to its implied probability and every outcome pays the same amount:
stakei = total × (1/di) ÷ Σ(1/dj) · profit = total × (1/Σ − 1)
Book A has the over at +105 (implied 48.78%), book B has the under at −102 (implied 50.50%). Total: 99.28% — an arb. On a $1,000 total, stake $491.36 at +105 and $508.64 at −102: either way you collect about $1,007.30, a guaranteed $7.30 (0.73%).
Real-world cautions: lines move while you're placing the second leg, books round stakes, and repeated arbing gets accounts limited. Treat small arbs (<1%) as barely worth the execution risk.
Deep dive: Arbitrage betting explained →
Constantly in small sizes — books move at different speeds. Most are under 1% and vanish in minutes; scanners that watch many books at once (like EdgeFinder's Pro arb view) are how people actually catch them.
Yes — it's just betting both sides at favorable prices. But most sportsbooks' terms let them limit or close accounts that arb systematically.
One leg moves before you place the other (leaving a normal bet), a book voids a leg, stake-rounding eats a thin margin, or a push rule differs between books on the same market.
Expected value of a bet from your fair win probability and the offered price.
Strip the bookmaker margin from two- or three-way odds to reveal the fair price.
Optimal bet size from your edge and bankroll, with half- and quarter-Kelly presets.