Arbitrage betting explained: why sure bets are rarely sure

By Barry Healy EditorPublished: 29 October 2026, 15:51 EDT8 min read

Last reviewed 29 Oct 2026 · Editorial policy

An empty racecourse at dusk, its pale oval track glowing under low amber light, with a row of tall floodlight masts casting long shadows across the grass and a few scattered blades of wet grass catching the last light.

How the stake maths of backing every outcome works, & why limits, voided bets & moving prices mean the profit depends on every bet standing.

What arbitrage betting is

Arbitrage betting, also called “sure bets” or sports arbitrage, means backing every possible outcome of one event with different betting companies so that, in theory, the result does not matter. According to Wikipedia’s entry on arbitrage betting, when conditions allow, a bettor placing one bet per outcome with different companies “can make a profit regardless of the outcome”.

Wikipedia says these opportunities arise either from bookmakers’ differing opinions on event outcomes or from errors. The profit also depends on every bet standing, meaning each is settled as it was placed. Much of this guide is about what happens when one does not.

A second version uses a betting exchange, an online platform where bettors can back an outcome or bet against it, with odds set by matching supply & demand. In a paper published in Economica in 2013, Egon Franck, Erwin Verbeek & Stephan Nüesch of the University of Zurich found cases where combining bookmaker odds with exchange odds “yields a positive return independent of the match’s outcome”.

Spotting an arb: implied probabilities under 100%

Implied probability is the likelihood of an outcome suggested by the quoted odds. With decimal odds, which show the total payout per unit staked, it is 1 divided by the odds. The Wikipedia article on bookmaking gives fractional odds of 6/4 as equivalent to decimal odds of 2.5 & as corresponding to an implied probability of 40%.

The Wikipedia article on the mathematics of bookmaking gives a football example. Odds of evens, 2–1 & 5–1 correspond to implied probabilities of 1⁄2 (50%), 1⁄3 & 1⁄6, adding up to a “fair book” of 100%.

A bookmaker then shortens the odds to make a profit. At 4–6, 6–4 & 4–1 the implied probabilities are 60%, 40% & 20%, a book of 120%. The amount above 100% is called the overround, also known as the bookmaker margin or “vig”, & it represents the bookmaker’s expected profit.

An arb is the reverse. Wikipedia says arbitrage occurs when the odds covering all outcomes have implied probabilities that “add up to less than 1”; Franck & his co-authors describe the sum as “less than unity”. Taking the best price for each outcome from different bookmakers is how such a total can arise.

Splitting the stakes: formula & worked example

To make every result pay the same amount, divide the return you are aiming for by the decimal odds of each outcome. This is the same as staking each outcome in proportion to its implied probability.

The bookmaking example on Wikipedia shows the arithmetic from the bookmaker’s side. A stake of £60.00 at 4–6, £40.00 at 6–4 & £20.00 at 4–1 each return exactly £100.00, so £120.00 taken in stakes pays out £100.00 whatever the result.

Wikipedia adds that “In reality, bookmakers use models of reducing that are more complicated than this model of the ‘ideal’ situation.”

Franck, Verbeek & Nüesch write that with an exchange, placing a bet on all possible outcomes “is no longer necessary to hedge against the uncertainty of the underlying outcome”, because a bettor “can simply buy favourable odds from a bookmaker and sell them directly on the bet exchange platform”.

According to their paper, when exploiting such opportunities bettors gain 1.6% on the bookmaker position & lose 0.2% on the hedging position from the exchange, on average.

Why arbs are small & short-lived

Wikipedia says arbitrage betting “involves relatively large sums of money, given that 98% of arbitrage opportunities return less than 1.2%”. It adds that the practice “is usually detected quickly by bookmakers”.

Franck, Verbeek & Nüesch studied 11,933 matches in the top five European leagues over seven seasons, 2004/05 to 2010/11, using odds from ten major bookmakers matched against the Betfair exchange. They found only 102 arbitrage opportunities between bookmakers where price differences exceeded commissions.

Combining a bookmaker with the exchange was far more common: they found 2287 such opportunities with an average return of 1.4%. The authors point to lower trading costs, noting that the exchange’s commission “lies between 2.0% and 5.0%” depending on a client’s activity, against an average bookmaker commission of 11.3% in their sample.

Risk one: account limits & closures

Wikipedia says arbitrage bettors “commonly see their maximum wager limits decreased heavily” once a sportsbook detects the behaviour, “often to the point of it no longer being viable”.

Regulator data from Great Britain shows how common restrictions are, though it covers all commercial restrictions, not only arbitrage. In a blog post on 23 July 2025, Gambling Commission chief executive Andrew Rhodes reported that, for 2024, operators restricted 643,779 of 14,923,840 active accounts in some form, a rate of 4.31 per cent.

The most common method was a stake factor, which caps an account’s maximum stake at a percentage of what an unrestricted customer may bet; it applied to 2.68 per cent of active accounts. Account closures for commercial reasons affected 2.23 per cent.

The Commission found 46.78 per cent of restricted customers were in profit, compared with 25.42 per cent of active customers overall. It also notes that people hold accounts with several operators, so account numbers are not the number of people affected.

Risk two: obvious errors & voided bets

The sports book house rules at Turning Stone Resort Casino in New York say “void” means a bet “is deemed invalid and will be refunded”, & that whether a bet is void is decided case by case. Separately, the same rules define an “Error” to include bets accepted “At odds that are materially different from those available in the general betting industry at the time the bet was accepted”, judged against a majority of operators.

An arb usually depends on at least one price standing apart from the rest of the market, which is the situation that clause describes. WCPO reported that DraftKings’ house rules allow it to void “bets placed at odds that are materially different from those available in the general market at the time the bet was placed.”

In Ohio, WCPO reported that sportsbooks voided more than $77 million in wagers in the 13 months after the state legalised sports betting in January 2023, 0.91% of the $8.5 billion gambled. According to Jessica Franks, spokeswoman for the Ohio Casino Control Commission, Ohio allows voids for obvious errors as long as sportsbooks follow their house rules & clearly convey the reason.

Rules differ by state. Ian Messenger of the Association of Certified Gaming Compliance Specialists told WCPO that “Massachusetts, Pennsylvania and New Jersey require sportsbooks to seek approval before they void bets”, & regulators in Connecticut & New Jersey rejected DraftKings requests to void winning bets from an NBA game on Oct. 24, according to WCPO.

For an arber, a void on one side is the core risk. Wikipedia notes that bookmakers’ dislike of arbing could in the past result in “half of an arbitrage bet being canceled”, leaving the other half as an ordinary bet that can lose.

Risk three: prices & stakes that change

The stake maths only works if both bets are accepted at the quoted prices. The Wikipedia article on bookmaking says modern online bookmakers use real-time data & risk algorithms to adjust odds continuously as wagers are placed.

Franck & his co-authors note that once a bookmaker bet is placed, its payout is tied to the odds taken & does not change with later prices. The exposure is the gap before the second bet is placed, when the other price can move or disappear.

Stakes can change too. Turning Stone’s rules say the sports book, depending on risk, “may accept the bet request placed by the guest at a lower limit than what is posted”. If one bet is cut, the stakes no longer balance & part of the position is unhedged.

Because margins are thin, Wikipedia says arbing “involves relatively large sums of money”, so a reduced stake or a small price move can remove the profit.

House rules & regulators

Turning Stone’s rules say the sports book “may refuse to accept any bets” at any time & for any reason, including to protect itself where a guest is “engaging in arbitrage”. The same clause covers using bots or other automated means.

In Great Britain, the Gambling Commission says “gambling operators are entitled to act in their commercial interests and manage liabilities” & that “It is not within our regulatory remit to mandate how individual operators manage their commercial liabilities.” It adds that “Being a successful bettor is not a protected characteristic in discrimination law.”

The Commission also says licensed operators should be transparent with customers about how, when & why an account might be restricted, & that it is exploring improvements to that communication.

In Massachusetts, InGame reported that limited bettors began receiving letters on June 1 explaining why, under new state regulations. One bettor’s letter from Fanatics cited “potential arbitrage positions”, while DraftKings letters cited “targeting perceived market inefficiency” & attempts to exploit latency in live market updates.

InGame reported that the Massachusetts Gaming Commission said its staff would review the communications sent to limited bettors.

Questions

What is a sure bet?

It is another name for an arbitrage bet. Wikipedia describes placing one bet on each outcome with different companies so the bettor can “make a profit regardless of the outcome”, provided every bet stands.

How big are arbitrage profits?

Usually small. Wikipedia says 98% of arbitrage opportunities return less than 1.2%, & Franck, Verbeek & Nüesch found bookmaker-exchange arbs averaging 1.4% across seven seasons of top European football.

Do bookmakers limit arbitrage bettors?

Wikipedia says arbers commonly see maximum wager limits cut heavily once detected. Turning Stone’s house rules allow it to refuse bets from a guest “engaging in arbitrage”, & a Fanatics letter in Massachusetts cited “potential arbitrage positions” as the reason for a limit.

Is arbitrage betting legal?

None of the sources used here describes arbitrage betting as unlawful. What they document is operators’ freedom to refuse or limit it, & the Gambling Commission, whose data covers British gambling consumers, says operators are entitled to manage their liabilities.

Can a sportsbook cancel one side of an arb?

Turning Stone’s house rules define bets accepted at odds materially different from the general betting industry as an “Error”, & WCPO reported that DraftKings’ house rules allow it to void “bets placed at odds that are materially different from those available in the general market at the time the bet was placed.” Wikipedia says bookmakers’ unfavourable view of arbitrage could in the past result in “half of an arbitrage bet being canceled”. Ian Messenger told WCPO that state rules on voids differ, with Massachusetts, Pennsylvania & New Jersey requiring approval.

Sources

Sources checked 29 October 2026. Information only. 18+.

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