How arbitrage betting works and where it goes wrong
Updated 13 Nov 2026 · 3 min read
An arbitrage, or sure bet, covers every outcome of an event at prices that leave a profit whichever one wins. The arithmetic is simple and correct. What goes wrong happens outside the arithmetic, and this guide covers both halves.
Why one bookmaker never offers it
Inside a single bookmaker the implied chances of a market add up to more than 100%: two sides at 1.91 make 104.71%. Covering both from 1,000 returns less than you staked and loses 45.00. That gap is the margin, explained in what bookmaker margin is.
How two bookmakers can create one
Bookmakers disagree. If one offers 2.10 on the first outcome and another offers 2.05 on the second, the two best prices together imply only 96.40%. The total is under 100%, so the margin across the pair is negative.
Prices shown as fractions or with a plus and minus sign are covered in how to read betting odds.
Splitting the stake so every outcome pays the same
Each outcome gets a share of the total in proportion to one divided by its price. From 1,000, 493.98 goes on the first outcome and 506.02 on the second. Either way the return is 1037.35.
The profit is 37.35, or 3.73% of the money staked. The arbitrage calculator does the split for two or three outcomes and says at once when the prices do not form an arbitrage.
What breaks a sure bet in practice
- A price moves between your two bets. If the second price falls to 1.85 after the first bet is placed, the pair implies 101.67% and that side now returns 936.14, a loss of 63.86.
- A bookmaker voids a bet placed at a price it posted by mistake, and the other half is left standing alone.
- Stake limits stop you from placing the full amount on one side.
- Two bookmakers settle the same event by different rules, for example on a retirement or an abandoned match.
- Accounts that bet this way get limited or closed.
Where the numbers stop being enough
A margin under 100% can be checked in seconds with the margin calculator, and a real one is small: a few percent before any slip. Most of the work is speed, limits and reading the rules, none of which a calculator sees.
A bet that is only likely to be good, not certain, is a different problem. There the question is how much to stake, which is what the Kelly calculator is for.
Questions about sure bets
How do I spot an arbitrage?
Add up one divided by each of the best prices. Under 100% there is one. Prices of 2.10 and 2.05 give 96.40%.
How much does a sure bet make?
Usually a few percent of the total stake. The example makes 37.35 from 1,000, a return of 3.73%.
Is arbitrage betting risk-free?
On paper, yes. In practice a moved price, a voided bet, a stake limit or a different settling rule can each turn the profit into a loss.
Do I need accounts at several bookmakers?
Yes. The prices that form an arbitrage come from different bookmakers, so each side is placed with a different one.
Tools for checking an arbitrage
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