Horse racing arbitrage: backing a fixed price and laying it on the exchange

Horse racing arbitrage is backing a runner at a bookmaker's fixed price and laying it on a betting exchange at a lower price, so the two bets leave about the same amount whichever horse wins. With illustrative prices, $100 at $5.50 laid at $5.00 keeps about $4.65 either way after an illustrative 6% commission, against $545.36 of your money tied up until the race settles.

That is the arithmetic on paper. A racing arb is two bets with two operators, both placed before the jump, and it holds only if the bookmaker accepts the back at the price shown, the lay is matched in full, and nothing before the jump settles the two bets differently. Arbs built only from bookmakers' back prices, which on a race means covering every runner, are a separate case, covered in arbitrage betting in Australia.

A worked racing arb: $100 at $5.50, laid at $5.00

With illustrative prices, Bookmaker A offers Runner 3 at $5.50 and the exchange shows $5.00 to lay. Commission is an illustrative 6%: the exchange sets its own rate and can change it.

1. Back $100 at $5.50 with Bookmaker A. If Runner 3 wins, it returns 100 x 5.50 = $550.
2. Lay stake = back stake x back price / (lay price - rate) = 100 x 5.50 / (5.00 - 0.06) = 550 / 4.94 = $111.34.
3. Liability = 111.34 x (5.00 - 1) = $445.36.

The two lines differ by 2 cents because the lay stake is rounded to the cent. The arb ties up the $100 stake with Bookmaker A and $445.36 of liability on the exchange, $545.36 in all, so its $4.65 is about 4.65 / 545.36 = 0.85% of the money committed.

Commission decides whether the arb exists at all:

The test is back price x (1 - rate) above lay price - rate: here 5.50 x 0.94 = 5.17 against 4.94. This arb disappears at a rate of (5.50 - 5.00) / (5.50 - 1) = 11.1%, and at 6% the back price has to beat (5.00 - 0.06) / 0.94 = 5.255, so $5.26 or more, to lock a result above zero against a $5.00 lay. The back lay calculator sizes the lay against your back at your own rate, and the lay bet calculator checks the liability it creates. The arbitrage calculator reads every price as a back bet, so it splits a stake across two to four back prices, at bookmakers or on an exchange with its commission typed in; a lay price typed into it shows an arb that is not there.

Why tote prices cannot be arbed

An arb needs two prices that are fixed when you bet, and a tote dividend is not one. The tote pools every bet of one type on a race and, after the operator's commission, splits what is left among the winning bets as a dividend, usually a return per $1, declared only after the result. The Northern Territory's tote rules, for one, set the payout as total bets less refunds less commission. Before the race you see only approximates, and they keep moving until betting closes.

That is a bet on the size of the dividend, not an arb. The same goes for any leg paid at a price set after you bet: a bet at a starting price, a bookmaker product that pays a tote dividend, or a lay at the exchange's own starting price. Only fixed prices on both sides can lock a result; tote betting explains how pools pay.

Price moves between the two bets

The two bets cannot go on at the same instant, and a racing price can move in the seconds between them. Place the bookmaker bet first, because it is the leg that can be refused, part accepted or re-priced, then lay at once for exactly what was accepted. With the $100 at $5.50 on and commission at 6%, the exchange price you finally lay at decides the result:

A 30-cent drift on the lay side turns the $4.65 arb into a loss. The bookmaker side moves too. If Bookmaker A accepts the bet only at $5.20, the $5.00 lay is 520 / 4.94 = $105.26, leaving 420 - 421.04 = -$1.04 if Runner 3 wins and 105.26 x 0.94 - 100 = -$1.06 if it loses.

Order matters most on a part acceptance. If Bookmaker A takes $40 of your $100, lay for the $40 alone: 220 / 4.94 = $44.53, keeping +$1.88 or +$1.86. Lay first for the full $100 and then get $40 accepted, and you hold 40 x 4.50 - 445.36 = -$265.36 if Runner 3 wins against -40 + 104.66 = +$64.66 if it loses. A lay can also be part matched, which leaves part of the back uncovered; unmatched bets covers what happens to the rest at the jump.

How a late scratching splits the two legs

A late scratching changes both legs, under two separate sets of rules:

• The bookmaker leg. A winning back struck before the scratching is cut by the deduction the Stewards declare, in cents in the dollar, and a back on the scratched runner itself is refunded. For NSW on-course bookmakers the deduction comes off the ticket's face value (BR 14(4) of Racing NSW's Rules of Betting); online bookmakers set their own method, and scratching deductions works both readings.
• The exchange leg. Bets on the scratched runner are void, and matched bets on the others can have their prices cut by a reduction factor the exchange's rules set for that runner, often only above a threshold. Waiting offers can be cancelled. Check your exchange's rules.

Nothing ties the two figures together. With the $100 at $5.50 and the $111.34 lay at $5.00, assuming the factor comes off the whole lay price, a late scratching after both bets does this (illustrative deductions and factors):

The losing column never moves: a factor changes the lay's price, not its stake, and a losing back loses the same $100. Every swing lands on the winning column. After any scratching, rework both legs before the jump from the declared deduction and factor, and resize the lay at the new price if the winning column has moved too far. Dead heats can split the legs the same way when the two operators' dead-heat rules differ, and arbitrage rule mismatches lists where settlement rules part company.

Bookmaker terms decide what is allowed in practice

The law and a bookmaker's terms are separate questions, and B337 gives no legal advice. Is arbitrage betting legal in Australia sets out what the law covers; for your own position, ask an independent lawyer. Bet only with providers on ACMA's register of licensed wagering providers: offshore operators are illegal in Australia, and their customers lose Australian protections. Use only accounts in your own name. On the practical side, each bookmaker's terms decide whether your arb survives:

• A bet can be refused, part accepted or accepted at a different price, which is why the back goes first.
• A bookmaker can void a bet under its terms, so read the clause on errors in prices before backing a price that looks out of line. A voided back leaves the lay on its own: -$445.36 if Runner 3 wins, +$104.66 if it loses, the position called a one-legged arb.
• A bookmaker can limit your stakes or close the account. Many restrict or prohibit automated betting, third-party access and multiple accounts, and that risk is yours.
• On some races, minimum bet limits set by state racing bodies, or in WA by its regulator, require bookmakers to accept fixed-odds bets to lose up to a set amount at the displayed price (in WA, an amount the punter can win), with conditions and exclusions, as minimum bet limits explains.

Racing arb mistakes, with what each costs

Where B337 fits

The Terminal shows prices from 40+ bookmakers across racing and sports beside Betfair back and lay, so on a race a bookmaker price above the exchange lay price is there to see, and the racing board's EV overlays compare each bookmaker's price with the exchange. B337's screeners, arbitrage included, are sports screeners: the arbitrage screener looks for opposing prices at different bookmakers across sports markets. B337 has no arbitrage strategy and does not place the lay or any other exchange bet, so both legs of a racing arb are yours to place, the lay in your own exchange account.

The board's prices are collected on a repeating cycle, so check both prices with the bookmaker and the exchange before either bet goes on. A free account opens a limited view of the Terminal with live odds, which does not include Betfair's prices or the EV overlays. Bookmaker names are trade marks of their owners. B337 is not affiliated with them. For laying itself, from liability to BSP, start at the lay betting hub.