Arbitrage Betting Calculator Australia: 2-Way, 3-Way and Racing Stakes Rounded to Whole Dollars

By the AUOdds team · Updated 8 Nov 2026 · 7 min read

Key takeaways

Every lock uses the same formula: add up 1 ÷ odds for each outcome, and if the total is under 1, the prices lock in a return at the prices shown.
Rounding stakes to whole dollars always costs some margin, so recheck the worst-case return after rounding, not before.
Three-way NRL and AFL markets and racing fields need one stake per outcome, and a long-priced draw or runner still has to be covered.
Per-book rules on draws, retirements, scratchings and stake limits can undo a lock, so check them before you place the first leg.
Real risks remain: prices move, stakes get limited, bets can be voided and mistakes happen.

An arbitrage betting calculator Australia punters can rely on has to do more than split a stake between two prices. Local locks come as two-way tennis matches, three-way NRL and AFL head-to-heads with a draw, and full racing fields. Bet slips are also easier to fill with whole dollars than with $245.14. This guide works through the maths for each type with realistic decimal odds and shows how much margin you give up when you round. It finishes with a per-book checklist. You can use it as a manual betting arbitrage calculator or as a standard to test any tool you're considering. The aim is simple: know exactly what each leg returns before you place it.

In this guide

What should an arbitrage betting calculator Australia punters trust actually do?
How does the 2-way maths work on a tennis match?
How do you calculate a 3-way NRL or AFL lock with the draw?
Why does rounding to whole dollars cost you margin?
How does a racing calculator handle a full field?
Which per-book rules should you check before placing each leg?
When does it make sense to skip the manual maths?

What should an arbitrage betting calculator Australia punters trust actually do?

Most free tools handle one job: two outcomes, two prices and two stakes to the cent. That works for a tennis head-to-head. Australian locks come in more shapes than that.

A useful arb betting calculator should cover:

2-way markets: tennis head-to-head, line and total markets.
3-way markets: head-to-head plus the draw, where the draw price is usually long.
Racing fields: one stake per runner, across every runner in the race.
Rounding: whole-dollar or $5 stakes, with every leg's return recalculated after rounding.
Rounding cost: the margin lost to rounding, shown in dollars and percentage points.

The core formula is the same for all of them. For each outcome, divide 1 by the decimal odds to get the implied probability, then add those figures up. If the total is under 1, the prices form a lock.

Each stake is then your total outlay multiplied by that outcome's share of the total: stake = outlay × (1 ÷ odds) ÷ sum. Whichever leg wins returns roughly outlay ÷ sum.

The best arbitrage betting calculator differs from a basic one in what happens next. As soon as you round, the legs no longer return identical amounts. The return that matters is the lowest one, so the calculator has to recheck every leg after rounding instead of quoting the tidy pre-rounding figure.

How does the 2-way maths work on a tennis match?

Take a pre-match tennis market with Player A at 2.10 at one bookmaker and Player B at 2.02 at another. That's the kind of gap you might see between Sportsbet, TAB or Neds style prices before a match starts.

Step 1: implied probabilities

1 ÷ 2.10 = 0.4762
1 ÷ 2.02 = 0.4950
Sum = 0.9712

The sum is under 1, so it's a lock. The margin is 1 ÷ 0.9712 = 1.0296, or about 2.96% on your outlay.

Step 2: exact stakes on a $500 outlay

Player A: $500 × 0.4762 ÷ 0.9712 = $245.14, which returns $514.79
Player B: $500 × 0.4950 ÷ 0.9712 = $254.86, which returns $514.82

Step 3: round and recheck

Player A: $245 returns $514.50
Player B: $255 returns $515.10

The worst case is now $514.50, a $14.50 return over outlay, or 2.90%. Rounding cost you about $0.31, or 0.06 percentage points. On a margin this wide, that cost hardly matters.

Two things to confirm before placing either leg. First, check that both prices are still on the bet slip. Second, check that each book settles the match the same way if a player retires. Retirement rules can differ between bookmakers, so read each book's terms rather than assuming they match.

How do you calculate a 3-way NRL or AFL lock with the draw?

Draws are rare in NRL and AFL, but where a market lists the draw as its own outcome, you have to cover it. Some head-to-head markets don't list a draw. In those, a draw is settled under that bookmaker's own rules, so check those rules before you treat the market as two-way.

Here's a three-way NRL example:

Home at 2.15 (Book A): 1 ÷ 2.15 = 0.4651
Away at 2.04 (Book B): 1 ÷ 2.04 = 0.4902
Draw at 34.00 (Book C): 1 ÷ 34 = 0.0294
Sum = 0.9847, a margin of about 1.55%

On a $600 outlay, the exact stakes are $283.40 home, $298.68 away and $17.92 draw. Each leg returns about $609.30.

Rounded to whole dollars, you'd bet $283, $299 and $18:

Home returns $608.45
Away returns $609.96
Draw returns $612.00

The worst case is $608.45, or $8.45 over outlay (1.41%). Rounding cost $0.85.

Now try rounding to $5 so the stakes look like everyday bets: $285, $300 and $20, for $605 outlay. Returns are $612.75, $612.00 and $680.00. The worst case is $7.00 over outlay, or 1.16%.

So $5 rounding cost almost 0.4 percentage points on this lock. Most of that went into overstaking the draw, which pays $680 if it lands but drags down your minimum return. A good calculator shows you this trade-off before you commit.

Why does rounding to whole dollars cost you margin?

Rounding moves each stake away from its exact share of the outlay. One leg ends up slightly overstaked and another slightly understaked. The understaked leg sets your worst-case return.

This matters more than it looks, because real locks are usually thin. AUOdds data puts the typical lock at 1.2% over the last 30 days and 1.9% over the last 7 days. On a $200 outlay at 1.2%, the whole margin is $2.40. A $0.80 rounding loss takes a third of it.

You can keep the loss down with a few habits:

Round the big legs to $1, not $5. Short-priced legs carry most of the outlay, so a $5 step there shifts the result most.
Keep long-priced legs exact to the dollar. On the draw or a long-priced runner, $2 too much is multiplied by the odds.
Test the neighbours. After rounding, try moving $1 from the leg with the highest return to the leg with the lowest. Keep whichever combination gives the higher worst-case return.
Let the outlay float. If $600 won't split cleanly, $598 or $603 might.
Set a floor. Decide in advance the minimum post-rounding margin you'll accept, and skip anything below it.

Round figures also cut typing errors. Entering $245 is far less likely to go wrong than $245.14, and a mis-keyed stake usually costs more than rounding ever will.

How does a racing calculator handle a full field?

A racing lock means taking the best fixed-odds price on every runner across different books, so that whichever horse wins, you're covered. Small fields are the most practical, because every extra runner is another bet to place before prices move.

Here's a four-runner field:

Runner 1 at 2.45: 0.4082
Runner 2 at 3.50: 0.2857
Runner 3 at 5.50: 0.1818
Runner 4 at 9.50: 0.1053
Sum = 0.9810, a margin of about 1.94%

On a $400 outlay, the exact stakes are $166.43, $116.50, $74.14 and $42.92. Each returns about $407.77.

Rounded to $166, $117, $74 and $43:

Runner 1 returns $406.70
Runner 2 returns $409.50
Runner 3 returns $407.00
Runner 4 returns $408.50

The worst case is $406.70, or $6.70 over outlay (1.68%). Moving $1 from Runner 2 to Runner 1 sounds sensible, but it drops Runner 2's return to $406.00. The first split is the better one.

The racing-specific risk is a late scratching. If a runner is scratched after you've backed it, that leg may be refunded or adjusted under the book's rules. Your remaining legs then no longer form the same lock. Each bookmaker sets its own rules on scratchings and deductions, so read them before you build a racing lock.

Also confirm that every leg is a fixed-odds win bet on the same race, not a mix of bet types.

Which per-book rules should you check before placing each leg?

The maths only holds if every leg is accepted at the price shown and settled the same way. Run this checklist before you place the first bet:

Licensed bookmaker. Only use Australian-licensed operators. The ACMA's 'Check if a gambling operator is legal' page lets you confirm this.
Same market, same settlement. Check that a 'head-to-head' at one book means the same thing at the other. Look for draw handling, extra time or golden point, and retirement rules.
Pre-match only. Online in-play betting isn't offered to Australians, so every lock here is placed before the start.
Price on the slip. Confirm that the odds on the bet slip match what you entered. If one has shortened, recalculate before going further.
Stake limits. A book may accept less than you ask. Place the leg most likely to be limited first. If it's cut, rescale the other legs to match.
Funds ready. Have money already deposited at each book. Waiting on a deposit is a common reason a price disappears.
Order of placement. Place the hardest or most volatile leg first, then the leg with the steadiest price.
Records. Note the time, odds, stake and book for every leg so you can check settlements later.

If a leg fails partway through, don't chase. Work out the position you're actually holding, then decide whether to cover it at the current best price or accept the exposure. Mistakes and voided bets are real costs, so count them in your results.

When does it make sense to skip the manual maths?

A manual calculator is worth learning. It shows you why a lock works and helps you spot errors. The weak point is speed. By the time you've compared prices across books, run the sums and rounded the stakes, a thin margin can be gone.

That's the trade-off with any betting arbitrage calculator. It checks prices you've already found, but it doesn't find them for you.

If you'd rather skip the maths, AUOdds sends the exact stakes for each outcome, already worked out. The prices come from about 15,000 tracked across 73 bookmakers, typically around 10 seconds old. You still check each price on the slip, place each bet yourself and apply the checklist above. Prices can still move between the alert and your bet slip, and limits or voids still apply.

A quick reminder before any of this: betting is for adults 18 and over. Set a budget you can afford to lose and stick to it. If betting stops feeling under control, BetStop, the National Self-Exclusion Register, lets you exclude yourself from all licensed Australian online and phone bookmakers in one step. Gambling Help Online offers free, confidential support at any hour. The Department of Social Services also lists support options on its gambling page.

Frequently asked questions

What is the best arbitrage betting calculator for Australia?

Look for one that handles two-way, three-way and racing markets in one place and lets you round stakes to whole dollars. After rounding, it should recheck every leg's return and show the worst case. If it only reports the pre-rounding margin, it can overstate what you'll actually receive. Test any tool against the worked examples in this guide.

Should I round arbitrage stakes to whole dollars?

Usually, yes. Whole-dollar stakes are quicker to enter and less prone to typing mistakes. Rounding does cost some margin, typically cents to a dollar on a few hundred dollars of outlay. Round in $1 steps on short-priced legs, keep long-priced legs tight and recheck the lowest return. If the post-rounding margin falls below your floor, skip the lock.

Do I pay tax on arbitrage betting returns in Australia?

The ATO's Taxation Ruling IT 2655 covers gambling and betting winnings, including the factors the ATO considers when deciding whether betting amounts to a business. The ATO also has a page on gambling winnings. Your situation depends on your own circumstances. If you're unsure, speak with a registered tax agent rather than relying on a general guide.

What happens if a price moves after I've placed one leg?

Recalculate using the new price before placing anything else. If the lock still holds, adjust the remaining stakes. If it doesn't, you're holding an open bet, so decide whether to cover it at the best available price or accept the exposure. That's why it pays to place the leg most likely to move or be limited first.

Sources

Check if a gambling operator is legal, Australian Communications and Media Authority
Taxation Ruling IT 2655: gambling and betting winnings, Australian Taxation Office
Crypto asset prizes and gambling winnings, Australian Taxation Office
BetStop, Australian Government
BetStop: the National Self-Exclusion Register, Australian Communications and Media Authority
Gambling, Department of Social Services

More guides

18+ only. Betting involves risk, and prices and limits can change before you place a bet. If gambling isn't fun any more, Gambling Help Online (1800 858 858) is free and confidential, and BetStop lets you exclude yourself from every licensed Australian bookmaker.