Arbitrage means you back every outcome of a match at different bookmakers, so you profit whichever one happens. It is possible when two companies disagree enough on the price.
The calculation
For two outcomes the sum of the inverse odds must be below 1.
Example
Bookmaker A offers 2.10 on Over 2.5 and bookmaker B offers 2.05 on Under 2.5. The inverses are 1 / 2.10 = 0.476 and 1 / 2.05 = 0.488, total 0.964. With 100 euros you stake 49.4 on Over and 50.6 on Under. Either way you receive about 103.7, a profit of about 3.7%.
Why it is hard
- Opportunities last minutes and the price moves before you place it.
- If you place one leg and the other is lost, you are left with a plain bet.
- Bookmakers restrict accounts that play only such bets.
- Void rules and different lines (2.5 against 2.75) change the result.
The profit is small and the risk of error is large. It is not as "sure" as it sounds.