Split your stake across outcomes for guaranteed profit when combined odds allow arbitrage.
An arbitrage exists when the implied probabilities across every outcome of a market, taken at the best available price for each, total less than 100%. Stake proportionally to those prices and every result pays the same amount, which is more than you put in.
The calculator splits the stake for you and shows the locked profit. The arithmetic is the easy part; the hard part is that these prices exist for minutes, and books limit or void accounts that harvest them.
Three things eat the edge: stake limits on the good price, the risk that one leg is voided while the other stands, and account restrictions once a book notices the pattern. Currency conversion and withdrawal fees can also swallow a 1-2% margin.
Treat a small arb as a break-even trade with operational risk, not as income.
An arb exists because two books disagree, and both of them are watching. Prices move, one leg gets taken before the other, and accounts that repeatedly take the wrong side of a moving line are limited or stake-factored down to nothing.
The practical risks are structural rather than mathematical: a voided leg leaves you with an unhedged bet, a rule difference between books can settle the same event two ways, and a rejected stake turns a guaranteed profit into an ordinary gamble.
Below roughly 1% the fees and voided-leg risk usually eat the profit. Large arbs almost always mean one book has an error or a different rule set.
It is not illegal, but it breaches most bookmakers’ terms and typically ends in limits or closure.