Sure betting explained: how arbitrage works

What a sure bet is

A sure bet, or arb, is a set of bets on every outcome of one event, placed at different bookmakers, where the odds are high enough that you finish ahead whichever outcome happens. You are not predicting anything. You are exploiting the fact that two prices disagree.

The edge per arb is small, usually 1% to 3% of the total stake. The appeal is that it is repeatable and low-variance. Low-risk is not the same as no-risk, though, and most of this guide is about the gap between the two.

Why odds disagree between bookmakers

Every bookmaker sets and moves its own prices. They use different models, they react to news at different speeds, and they balance their own liabilities. A soft bookmaker with lots of recreational customers might shade the popular side, while a sharp book moves its line within seconds of informed money arriving.

For a few minutes, sometimes longer, one book can be out of step with the rest of the market. If one book is too generous on outcome A and another is too generous on outcome B, the two prices together can add up to less than 100% implied probability. That gap is the arb.

The 1/odds test and arb percentage

Decimal odds convert to implied probability with one division. If your odds are in fractional or American format, convert them first with the odds converter.

implied probability = 1 / decimal odds

Add the implied probabilities of the best price for each outcome. A normal market adds up to more than 1, because the bookmaker's margin is built in. If the sum is below 1, you have a sure bet.

S = 1/oddsA + 1/oddsB (+ 1/oddsC)
S < 1 → arbitrage exists

There are two common ways to quote the size of the edge: profit as a share of the total stake, or return on the money staked. ArbVantage shows the second, which is what most arb scanners quote.

profit % of total stake = (1 − S) × 100
return on stake % = (1/S − 1) × 100

The difference is small for typical arbs but worth knowing when you compare numbers between tools.

Splitting your stakes on a 2-way market

The goal is an equal payout whichever side wins. You get that by staking each leg in proportion to its implied probability:

stake A = total × (1/oddsA) / S
stake B = total × (1/oddsB) / S
payout = total / S

In practice you will often round stakes to look less like an arber, for example €495 and €505. The payouts become €1,039.50 if A wins and €1,035.25 if B wins. Both are still profitable, just uneven. The arbitrage calculator shows you exactly what rounding does before you place anything.

A 3-way example: football 1X2

Football match result markets have three outcomes: home, draw, away. The method is identical with a third term.

S = 1/oddsH + 1/oddsD + 1/oddsA
stake X = total × (1/oddsX) / S

Say the best prices are home 2.45 at one book, draw 3.60 at a second and away 3.30 at a third, with €500 to spread.

S = 0.40816 + 0.27778 + 0.30303 = 0.98897, so the edge is about 1.10% of total stake (a 1.12% return).

Three legs means three chances for something to go wrong, and €5.58 is not much of a cushion. That is typical: the margins are thin, so execution matters more than anything else.

The real risks

Arbitrage removes outcome risk on paper. These are the things that put it back.

• The price moves before your second leg. This is the most common problem. You get on at 2.10, then the other book drops from 2.05 to 1.90. Now you either accept a small loss or leave one side open. Place the leg most likely to move first, usually the soft book with the outlying price.
• Palpable errors and voids. If one price was an obvious mistake, the bookmaker may void that bet under its terms. You are then left holding an unhedged bet at the other book. Read our guide to palpable errors before chasing very large arbs.
• Different settlement rules. Bookmakers disagree on tennis retirements (some settle after one set, some after one ball, some void), on whether a market includes overtime or extra time, and on dead-heat rules. Two legs settled under different rules can both lose.
• Stake limits. A book may accept only part of your stake, which unbalances the arb. Profitable accounts also get restricted over time. See how bookmaker limits work.
• Rounding and minimums. Rounding, minimum stakes and currency conversion all shave the margin. On a 1% arb, a few euro of rounding can wipe it out.
• Capital and admin. Money sits in many accounts and withdrawals take time. The bankroll guide covers how to size and split it.

Betting exchanges help with several of these: they rarely restrict winning customers and let you lay an outcome instead of backing the other side. The trade-off is commission. The exchanges guide explains how to include it in your maths.

Placing an arb: a practical checklist

1. Check it is the same event, the same market and the same line (for example both legs are "including overtime").
2. Check the settlement rules at both books for this sport, especially retirements and voids.
3. Confirm both prices are still live and the edge is above your minimum, say 1%.
4. Work out stakes with a calculator and decide on rounding before you start.
5. Place the most fragile leg first, usually the soft book with the outlying price.
6. Confirm it was accepted in full at the expected odds, then place the other leg straight away.
7. Record both bets so you can reconcile settled results and spot rule differences later.

Only stake money you can afford to lose. Mistakes happen to everyone, and one bad void can cost more than ten good arbs earn.