A surebet, also called an arbitrage bet, happens when two or more bookmakers price the same event so differently that backing every outcome still leaves a margin. The test is simple: convert each price to an implied probability by dividing 1 by the decimal odds, and add them up. If the total is below 100 percent, the prices contain a surebet. If it is above 100 percent, they do not, and the excess is the bookmakers' margin.

An arbitrage calculator does two jobs. First it runs that test. Then, if the test passes, it splits your total stake across the outcomes so that every outcome returns roughly the same amount. That split is the part people get wrong by hand, because equal stakes do not give equal returns when the odds differ. The stakes must be weighted by the implied probabilities.

Here is a clean worked example. Two bookmakers both offer 2.10 on opposite sides of a two-way market. Each price implies 1 divided by 2.10, which is about 47.62 percent. Added together they make 95.24 percent, which is below 100 percent, so the arbitrage margin is 100 minus 95.24, or about 4.76 percent. With a total stake of 100 units, the calculator splits it evenly here, 50 units on each side, because the odds are equal.

Each winning side then returns 50 times 2.10, which is 105 units. You staked 100 units in total, so the profit is 5 units whichever side wins. That is the maths the calculator performs: stake on each outcome equals your total stake multiplied by that outcome's implied probability, divided by the total implied probability. The return on any outcome is then your total stake divided by the total implied probability.

When the odds differ, the split differs too. Take prices of 2.15 and 2.05. Their implied probabilities are about 46.51 percent and 48.78 percent, totalling 95.29 percent, a margin of about 4.71 percent. On a 100-unit total stake, the calculator puts about 48.82 units on the 2.15 side and 51.18 units on the 2.05 side, and either side returns about 105 units. The side with the longer price gets the smaller stake, because it pays more per unit.

A calculator shows a theoretical margin, not a guaranteed profit in practice. Prices move, and the window can close in seconds. A bookmaker may limit or reject a stake, one leg may be voided, or the two prices may cover slightly different markets with different settlement rules. If any leg fails to be placed at the entered price, the maths no longer holds, and a surebet can turn into an ordinary losing bet.

To use an arbitrage calculator well, enter prices from one complete market, use the same currency throughout, and recheck the prices immediately before placing anything. Treat the margin as a check, not a promise: it tells you whether the prices, as entered, contain an edge, and how to split a stake if you decide to act on them.

Q: What makes a surebet? A: When the implied probabilities of all outcomes in one market add up to less than 100 percent. The gap below 100 percent is the arbitrage margin.

Q: Why are the stakes not equal? A: Because each stake must be weighted by its implied probability so that every outcome returns the same total. Equal stakes only work when the odds are equal.

Q: Does a calculator guarantee profit? A: No. It shows the theoretical margin on the prices you entered. Odds movement, stake limits, voided legs or mismatched markets can all break the result in practice.