How bookmaker margins work
A bookmaker margin is the built-in commission that an operator adds to a market so that the combined odds represent more than 100% probability. That extra slice — often called the overround or vig — is why you can't win long term simply by backing every runner. On this page you'll learn how to calculate a margin from decimal odds, see a fully worked example, and compare the average margins we sampled across Australian bookmakers. Lower margin means better value on the same result, so understanding it is the single most useful skill for getting more back per bet.
What the margin actually is
Every price a bookmaker posts implies a probability. Convert a decimal price to implied probability by dividing 1 by the odds. A fair two-way market — say Heads or Tails — would price both sides at 2.00, giving 50% + 50% = 100%. No margin, no profit for the house.
Real markets never add to 100%. Add up the implied probabilities of every outcome in a market and you'll get something above 100%. The amount above 100% is the margin. If a head-to-head market totals 105%, the margin is 5%. That 5% is the operator's theoretical edge over a balanced book.
The more selections in a market, the more places the operator can hide margin. A two-runner tennis match carries margin across two prices; a 20-runner Melbourne Cup field spreads it across twenty. That's why racing and large outright markets typically carry a chunkier overround than a simple head-to-head.
The formula, step by step
For a market with prices O1, O2 ... On, the margin is:
Margin % = (1/O1 + 1/O2 + ... + 1/On) − 1
- Convert each price to implied probability (1 ÷ decimal odds).
- Add them together to get the "book percentage".
- Subtract 1 (or 100%) — what's left is the margin.
The smaller that final number, the sharper the price and the more the market pays back to bettors as a group. A margin of 0.05 means the book totals 105%; a margin of 0.075 means 107.5%.
Implied Probability Calculator
A worked example on an NRL head-to-head
Take an NRL match priced two ways. Say a bookmaker posts:
- Team A: 1.80
- Team B: 2.10
Implied probabilities:
- 1 ÷ 1.80 = 0.5556 (55.56%)
- 1 ÷ 2.10 = 0.4762 (47.62%)
Book percentage = 55.56% + 47.62% = 103.18%.
Margin = 103.18% − 100% = 3.18%.
Now compare a second bookmaker on the same match at 1.85 and 2.05:
- 1 ÷ 1.85 = 0.5405
- 1 ÷ 2.05 = 0.4878
Book = 108.3% ... wait, add carefully: 54.05% + 48.78% = 102.83%, margin 2.83%. The second book is fractionally sharper.
What does the difference cost you in dollars? Back Team A for $100. At 1.80 you'd collect $180 (a $80 profit). At 1.85 you'd collect $185 ($85 profit). Same result, same stake — $5 more in your pocket purely because the second operator ran a tighter market. Across a season of bets that gap compounds, which is why margin matters far more than a one-off promotion.
Fair odds vs the price you're offered
You can strip the margin out to estimate a "fair" price. Using the first NRL example, divide each implied probability by the book percentage (1.0318) to normalise back to 100%:
- Team A fair probability: 55.56% ÷ 1.0318 = 53.85% → fair odds ≈ 1.857
- Team B fair probability: 47.62% ÷ 1.0318 = 46.15% → fair odds ≈ 2.167
The difference between the fair odds (1.857) and the offered odds (1.80) is the operator's take on that selection. This is also why comparing prices across bookmakers is worthwhile: if one book's actual price beats another book's fair price, you've found genuine value.
How sampled margins compared across Australian bookmakers
The figures below are average margins from our odds sampling. They are drawn from different sample sizes, so treat smaller samples as indicative rather than definitive — a 30-price sample tells you less than a 250-price sample.
- Sportsbet: 4.91% average margin (44 prices sampled)
- PointsBet: 5.64% (51 sampled)
- Ladbrokes: 5.82% (258 sampled)
- Unibet: 5.84% (243 sampled)
- Neds: 5.89% (59 sampled)
- PlayUp: 5.93% (44 sampled)
- TAB: 6.05% (59 sampled)
- betr: 6.19% (56 sampled)
- bet365: 7.44% (30 sampled)
On the sampled data, Sportsbet posted the lowest average margin, while bet365 posted the highest — though bet365's 30-price sample is the smallest here and shouldn't be read as a settled verdict. Ladbrokes and Unibet, with the two largest samples, both sat around 5.8%, which is a reasonable benchmark for a broad Australian book.
A separate metric worth watching is best-price share — how often an operator held the top price in a sampled market. Unibet (56.0%) and Ladbrokes (54.7%) topped the best-price counts in our sampling, with Neds at 35.6%. Best-price share and average margin measure slightly different things: one is about how often a book leads, the other about the overall tightness of its book.
Why margins vary by sport and market
You'll rarely see one flat margin across a bookmaker. Expect these patterns:
- Big head-to-heads (AFL, NRL, Premier League, ATP/WTA singles) usually carry the tightest margins because they're heavily shopped and easy to compare.
- Multi-runner racing markets — a full Horse Racing field — spread margin across many runners, so the overround is typically higher than a two-way price.
- Niche and prop markets (player props, exotic Bet Builder legs, futures) tend to carry more margin because they're harder to price and less frequently compared.
- Live Betting prices can move margin around quickly as the operator manages risk in-play.
If you build multis, remember the margin compounds: each leg carries its own overround, so a four-leg multi stacks four margins on top of each other. That's the mathematical reason singles on tight head-to-heads generally return more over time than long multis.
How to use margin knowledge when picking a bookmaker
Margin is only one factor, but it's the one that directly affects your returns on every settled bet. Here's how to weigh it against the rest:
- Chase value on your core sport: If you bet mostly AFL or NRL head-to-heads, favour books that ran tighter sampled margins and higher best-price share — Unibet and Ladbrokes led our best-price counts.
- Don't ignore the whole package: A slightly higher margin can be offset by a strong app, fast withdrawals (all listed operators show 0–24h processing) or PayID support for quick top-ups.
- Line-shop before you bet: Keep two or three accounts and take the best price on the day. The $5-per-$100 gap in our NRL example is the difference line-shopping captures.
- Match features to your style: If you lean on Cash Out or Bet Builder, confirm the operator supports them — Sportsbet, Ladbrokes, Unibet and bet365 list Bet Builder; PointsBet, Neds and betr list Fast Payout instead.
All operators covered here are licensed for Australian wagering under state and territory authorities (most under the NTRC, TAB under state authorities), and the minimum betting age is 18. Licensing is a safety baseline, not a value indicator — margin and price are separate questions from whether an operator is legal and regulated.
FAQ
What is a bookmaker margin in simple terms?
It's the operator's built-in commission. When you add up the implied probabilities of every outcome in a market, the total sits above 100%. The amount above 100% is the margin — the theoretical edge the bookmaker holds over a balanced book.
How do I calculate the margin from decimal odds?
Divide 1 by each price to get its implied probability, add them all together, then subtract 1. For a two-way market at 1.80 and 2.10, that's (1/1.80) + (1/2.10) − 1 = 0.0318, or a 3.18% margin.
Which sampled bookmaker had the lowest average margin?
In our sampling, Sportsbet showed the lowest average margin at 4.91% across 44 prices, ahead of PointsBet at 5.64%. bet365 showed the highest at 7.44%, though from the smallest sample (30 prices), so read it cautiously.
Is a lower margin always better?
For the value you get on a given result, yes — a tighter margin means a bigger payout on the same stake. But it's one factor among several. App quality, withdrawal speed, payment options like PayID, and whether you get the best price on the day all matter too.
Why do multis have effectively higher margins?
Because each leg carries its own overround. A four-leg multi stacks four margins on top of one another, so the combined price is further from fair value than any single leg would be. That's why singles on tightly-priced head-to-heads usually return more over time.
What's the difference between margin and best-price share?
Average margin measures how tight a book's overall pricing is. Best-price share measures how often an operator held the top price in sampled markets. Unibet (56.0%) and Ladbrokes (54.7%) led best-price share in our sampling, which is a different signal from overall margin tightness.