Arbitrage betting: how surebets work

Every decimal price implies a probability: 1 divided by the odds. A bookmaker's prices on one market add up to more than 100%, and the excess is its margin. Take the best price on each outcome from different bookmakers instead, and the total usually drops close to 100%. When it falls below 100%, the market is an arbitrage, and the gap below 100% is roughly your profit as a share of the total stake.

The stake on each outcome is set in proportion to its implied probability, so every outcome returns the same amount. That equal return is what makes the result irrelevant.

A tennis match, two outcomes. Bookmaker A offers 2.10 on Player 1; bookmaker B offers 2.05 on Player 2. The implied probabilities are 47.62% and 48.78%, together 96.40%: under 100%, so this is an arbitrage. With a total stake of 100 units:

Whoever wins, the bets return about 103.73 units for 100 staked, a profit of 3.73%. The formula behind it: stake on an outcome = total stake × (1 / its odds) / (sum of 1 / odds). The same works for three-way markets such as a football match result, with three bookmakers and three stakes.

Real stakes are usually rounded, which moves the returns by a few hundredths and can shrink a thin margin. If one leg is at a betting exchange, the price after commission is the one that counts, and the arbitrage may not survive it.

Every league page has an arbitrage tab. It compares each outcome of every market across 490+ bookmakers and exchanges, lists the combinations whose best prices add up to under 100%, and shows the size of the gap and a stake split for each. Exchange prices are shown net of commission where it applies. The tab is free with an account; the list is as of when you open it, so press Refresh before you act.

The underlying comparison is the same one on every match page: the bookmakers' prices for the same market, side by side, every bookmaker with a free account. If you would rather back one side at a price that beats the fair line than cover every outcome, the value bets list does exactly that, and the no-vig calculator shows how a fair price is worked out.

Price moves between legs. Arbs exist because one price is out of line, and out of line prices get corrected, often within seconds. If one leg is accepted and another is rejected, re-priced or limited, the remaining position is no longer covered and can lose money, and the new prices may not support the calculated margin.

Limits and restricted accounts. Bookmakers profile customers who always take their outlying prices, and cut their maximum stakes or close the account. That, more than anything, sets how long arbitrage lasts for any one bettor.

Palps. A price that is obviously wrong may be voided under the bookmaker's palpable error rules, after the other leg has already been placed. An unusually large arb is more often a mistake than a gift.

Different settlement rules. Two books can settle the same market differently: a tennis retirement, overtime in or out, dead heats, a postponed match. When one leg is voided and the other stands, the arbitrage is gone. Each bookmaker publishes its settlement rules per sport.

Small margins, real costs. A 2% arb can be eaten by payment fees, currency conversion or exchange commission. They come off the margin whichever result lands.