What an arb is
Two venues price the same match differently. If the gap is big enough, you back both sides and get paid whoever wins.
Most of the time the gap is boring. One bookmaker hasn’t updated its line yet. An order book on a prediction market hasn’t been bought up. Someone’s odds are simply a minute behind everyone else’s. Hundreds of matches are played every day, each with dozens of markets, so these gaps keep showing up.
An arbitrage bet (we just say arb, some people say surebet) is what you do with that gap. You stake on both outcomes at once, on different venues, and keep the difference whoever wins. There’s no “guessed right or wrong”. You’re not rooting for anyone, you’re covering both possible results and pocketing the gap. It’s less a bet than a bit of arithmetic.
On real numbers
A match with two outcomes, A wins or B wins, no draw. Bookmaker 1 offers 2.10 on A. At the same moment bookmaker 2 offers 2.10 on B. You put $100 on each.
Bookmaker 1 A wins 2.10 stake $100 Bookmaker 2 B wins 2.10 stake $100 ────────────────────────────────────────── on the table $200 A wins → 100 × 2.10 = $210 +$10 B wins → 100 × 2.10 = $210 +$10
The result of the match doesn’t matter to you at all. Either way you’re up $10, 5% on top. That’s an arb. Real gaps are usually smaller and the stakes are rarely equal, but the idea doesn’t change.
The obvious next question: how do you tell from the prices alone, before betting anything, whether there’s an arb or not? That’s the next page, Odds, probability and margin.