What a sure bet is
If the best odds on all the outcomes, taken from different bookmakers, imply less than 100% in total, you can bet on every outcome so that you win whatever the result.
An example: a home win at 2.10 with one bookmaker, a draw at 3.60 with a second, an away win at 4.20 with a third. The probabilities are 47.6% + 27.8% + 23.8% = 99.2%. Split the stakes in the same proportion (47.6 + 27.8 + 23.8 = 99.2 units), and any outcome returns 100: a profit of about 0.8%.
What history shows
At the best closing prices, 40% of matches formally had a sure bet, but the average profit was 1.4% of the total stake, and sure bets of 2% or more turned up in 9.2% of matches. At early prices there are fewer: 18% of matches, 0.7% on average.
An important caveat: the best prices were collected from dozens of bookmakers after the fact. Some of them lasted seconds; some were mistakes the bookmaker would cancel. There are far fewer real sure bets that you could get on in time.
Why it is hard
- You have to bet with several bookmakers at once, before the odds change.
- The profit is a fraction of a per cent, so you need big stakes — and big stakes mean limits.
- Bookmakers spot “arbers” by their betting patterns and cut their limits to token amounts.
- A bookmaker is entitled to void a bet at obviously wrong odds, and then the sure bet turns into an ordinary bet with risk.