Every price a sportsbook offers carries a hidden charge. It is not a fee you pay at checkout; it is baked into the odds themselves. That built-in charge is the bookmaker margin, and the maths behind it is called the overround. Understanding it is the difference between knowing what a price really says about a result and simply taking the number on the screen at face value. This guide explains what the margin is, how to work it out from decimal odds, how it changes the money you get back, and how it varies between markets and operators.
Key Points
- Bookmaker margin is the operator’s profit built into the odds — also called the vig, juice or hold.
- Overround is the amount by which the implied chances of all outcomes add up to more than 100%.
- Implied probability = 1 ÷ decimal odds × 100. Add every outcome, subtract 100 to get the overround.
- A tighter book (closer to 100%) returns more of your stake over time; a wider book costs you more.
- Margins stack on multis and run widest on niche and novelty markets.
- No knowledge of the margin guarantees a profit — it exists so the operator keeps a long-run edge.
What is the bookmaker margin?
A bookmaker margin is the profit an operator expects to make on a market, expressed as a percentage and built directly into the odds. In a perfectly fair market, the prices for every possible outcome would reflect the true chance of each result, and the implied chances would add up to exactly 100%. A sportsbook shortens those prices slightly so the chances add up to more than 100%. The surplus is the margin, sometimes called the “vig”, “juice”, “cut” or “hold”.
Because the charge is spread across all outcomes, it applies whether you back the favourite or the underdog. The bookmaker does not need to predict the result correctly to profit; if the money it takes is balanced across outcomes, the margin is its theoretical return no matter who wins. That is the core reason a sportsbook is a business rather than a coin toss. If you are new to how prices are set, our explainer on how sports betting works in New Zealand covers the wider picture.
The overround: how the numbers add past 100%
“Overround” is simply the technical name for the amount by which the implied chances in a market total more than 100%. A market with a 105% book has a 5% overround; a tighter market at 102% has a 2% overround. The closer the total sits to 100%, the better the value for you, because less of the price is being taken as the operator’s edge.
Turning odds into implied probability
The first step is to convert each price into an implied probability — the chance the odds are quietly claiming. For decimal odds, the formula is straightforward:
- Implied probability (%) = (1 ÷ decimal odds) × 100
So odds of 2.00 imply a 50% chance (1 ÷ 2.00 = 0.50). Odds of 1.50 imply 66.7%, and odds of 4.00 imply 25%. The shorter the price, the higher the implied chance.
Calculating the overround and the margin
Once you have the implied probability for every outcome, add them together. Then:
- Overround (%) = (sum of all implied probabilities) − 100
- Margin / hold (%) = overround ÷ (100 + overround) × 100
The overround tells you how far the book is inflated above a fair 100%. The margin (or hold) restates that as the share of total stakes the operator keeps if betting is balanced — a slightly smaller figure, because it is measured against the inflated total rather than a fair 100%. Both describe the same edge; they are just two ways of expressing it.
A worked example: two-way and three-way markets
Take a two-way market, such as a tennis match with no draw, priced at 1.90 on each player. Each side implies 1 ÷ 1.90 = 52.63%. Added together, the book is 105.26%, so the overround is 5.26% and the margin is about 5.0%. In a genuinely even contest the fair price would be 2.00 a side (50% each); the operator has shaved both prices to 1.90 to create its edge.
Three-way markets, such as a football result (home, draw, away), work the same way with an extra outcome. A tightly priced match might show 2.40, 3.20 and 3.40. Those imply 41.67%, 31.25% and 29.41%, a total of 102.33% — an overround of roughly 2.33%, a figure consistent with the worked example published by Sporting Life. A recreational book on the same game might offer 2.10, 3.30 and 3.50, implying 47.62%, 30.30% and 28.57%: a total of 106.49% and an overround of about 6.5%. Same match, very different edge. The table below sets these out side by side.
Margin Examples
| Market | Decimal odds | Implied probabilities | Book total | Overround |
|---|---|---|---|---|
| Two-way (e.g. tennis), tight | 1.90 / 1.90 | 52.63% / 52.63% | 105.26% | ~5.26% |
| Three-way (football), tight | 2.40 / 3.20 / 3.40 | 41.67% / 31.25% / 29.41% | 102.33% | ~2.33% |
| Three-way (football), recreational | 2.10 / 3.30 / 3.50 | 47.62% / 30.30% / 28.57% | 106.49% | ~6.49% |
| Fair book (no margin) | 2.00 / 2.00 | 50.00% / 50.00% | 100.00% | 0% |
Implied probability = 1 ÷ decimal odds × 100. Figures rounded; a lower book total means a smaller edge for the operator.
Why the margin matters for your returns
The margin is the single biggest structural reason most bettors lose over time. It acts like a tax on every bet, quietly lowering the price below the true odds of the event. A few points of margin may look trivial on one wager, but it compounds relentlessly across hundreds of bets.
It compounds within a single slip, too. The margin applies to each selection in a multi (accumulator), so the overrounds stack. A match market with a 2.3% overround becomes roughly 4.7% across a two-leg double and around 11.7% across a five-leg accumulator, because each added leg layers another edge on top. This is why multis are far more profitable for the operator than for the bettor, even when every individual price looks fair.
Understanding this is the foundation of value betting: to come out ahead over the long run, you need to find prices where your own estimate of the true chance beats the chance implied by the odds — by enough to clear the margin first.
How margins vary by market and bookmaker
Margins are not fixed. They shift with the type of market, the operator’s pricing model and how much risk the book is willing to carry. As a rough guide:
- Headline, high-liquidity markets (major football match results, big tennis matches) tend to carry the tightest margins, because operators compete hardest on them and the prices are easy for bettors to compare.
- Niche and speculative markets (first goalscorer, exact score, novelty and player-prop markets) usually carry much wider margins, because they are harder to price and harder to compare.
- Longer multis and same-game combinations stack margin with every added leg, as shown above.
Operators also differ. Low-margin, high-turnover books aim to take a small slice of a large volume of bets, while recreational books often run wider margins. Academic analysis of large UK operators has found match-market margins falling over the past two decades as online competition has intensified, but the gap between the sharpest and the softest prices on the same event remains real and worth checking. For a deeper look at how the prices themselves are framed, see our guide to sports betting odds explained.
How to find and compare margins
You do not need special software to spot a heavy margin. The method is always the same:
- Convert every price in the market to an implied probability with 1 ÷ odds.
- Add them up.
- Subtract 100 to get the overround.
Repeat this across two or three operators for the same event and the differences become obvious. A market at 102% is returning far more of your stake over time than the same market at 108%. Over a season, consistently taking the tighter book is one of the few edges fully within your control. To strip the margin out and see the “fair” chance an outcome is being given, divide each implied probability by the total — that rescales the book back to 100%.
Common mistakes bettors make about margins
- Treating odds as the true probability. The implied chance always overstates the real chance, because the margin is baked in. Fair odds are always a little longer than the price you see.
- Assuming a low margin means you will win. A tighter margin improves your long-run return on winning bets; it does not change which side actually wins. It is a cost, not a prediction.
- Ignoring the margin on multis. Stacking several “fair-looking” legs can hide a double-digit combined overround.
- Chasing novelty markets for value. These usually carry the widest margins, so apparent “specials” are often the worst-priced bets on the board.
- Comparing odds formats instead of margins. The format (decimal, fractional, moneyline) is cosmetic; the overround is what actually costs you.
The New Zealand context
In New Zealand, TAB NZ is the country’s domestic betting agency, offering both tote (pooled) and fixed-odds wagering on racing and sport. With fixed-odds bets, the margin works exactly as described above: it is built into the quoted price. With tote betting the mechanics differ — returns depend on how the pool is shared after a deduction is taken — but a deduction still reduces what is paid out. According to Department of Internal Affairs figures, New Zealanders’ spending (the amount lost) on TAB racing and sports betting was around NZ$371 million in 2023/24, a scale that reflects how margins add up across a market.
The regulatory picture is also shifting: a separate online casino licensing regime under the Online Casino Gambling Act 2026 is being rolled out, with the Department of Internal Affairs issuing first licences expected in 2027. Fixed-odds sports and racing betting sit under different rules. Before placing any bet, it is worth understanding the current sports betting laws in New Zealand and using the responsible gambling tools available to keep spending in check.
Responsible gambling and factual information
This article is general, factual information about how bookmaker margins and the overround are calculated. It is educational only: it does not encourage betting, does not constitute financial or betting advice, and no understanding of margins can guarantee a profit — the margin exists precisely so the operator holds a long-run edge. Betting should only ever involve money you can afford to lose.
If gambling is causing you or someone you know harm in New Zealand, free and confidential help is available 24/7 from the Gambling Helpline Aotearoa on 0800 654 655, or by texting 8006. Support is also available through the Department of Internal Affairs, the gambling regulator. You must be 18 or over to bet in New Zealand.
Reference sources
Frequently Asked Questions (FAQ)
What is a bookmaker margin?
It is the profit an operator builds into its odds by shortening the prices so the implied chances of all outcomes add up to more than 100%. The surplus above 100% is the margin, also called the vig, juice or hold. It applies to every outcome, so the bookmaker profits over time regardless of which side wins, as long as the money it takes is balanced.
How do I calculate the overround from decimal odds?
Convert each price to an implied probability using 1 divided by the decimal odds, multiply by 100, then add the results for every outcome. Subtract 100 from that total to get the overround. For example, two prices of 1.90 each imply 52.63% apiece, a total of 105.26%, so the overround is 5.26%.
What counts as a low or high margin?
On major, heavily traded match markets, overrounds of roughly 2% to 5% are common and considered tight. Recreational books and niche or novelty markets can run 7% or higher. The lower the figure, the more of your stake is returned over the long run, so comparing overrounds across operators on the same event is worthwhile.
Does a lower margin mean I will win more often?
No. The margin is a cost, not a prediction. A lower margin improves the price you receive and therefore your long-run return on winning bets, but it does not change the actual outcome of any event. Winning still depends on results going your way at a price that beats the true chance.
How can I reduce the impact of the margin?
Compare the overround across several operators and take the tightest price on the market you want, stick to high-liquidity markets where margins are smaller, and avoid long multis and novelty bets where margins stack or run wide. Removing the margin from the odds to estimate the fair chance also helps you judge whether a price offers genuine value.
