Bookmaker Overround: How the Built-In Margin in the Odds Works
Overround (bookmaker margin, also called “vig”) is the single most important number in the entire mechanics of sports betting — and, at the same time, the one almost nobody shows with a full, checkable calculation. This article does exactly that: it takes a real, example 1X2 market, calculates its overround step by step, and shows exactly where that surplus above 100% ends up — not in theory, in actual zloty. It’s a built-in, structural cost of play, independent of whether you win or lose any single bet — which is why understanding it matters more than chasing a single “sure thing”. Updated: September 2026.
Starting point — implied probability
Overround is calculated from implied probabilities, i.e. the value 1 ÷ odds for every possible outcome. We explain the full formula and give separate single-price examples in implied probability — here we go one step further and sum those values for the whole market, not a single price.
A fully worked example — a 1X2 market
An example, purely illustrative match-result market, with no reference to any real fixture: odds for a home win of 2.10, a draw of 3.40, an away win of 3.60. We calculate the implied probability of each outcome:
- Home: 1 ÷ 2.10 = 0.476190… ≈ 47.62%
- Draw: 1 ÷ 3.40 = 0.294118… ≈ 29.41%
- Away: 1 ÷ 3.60 = 0.277778… ≈ 27.78%
Total: 47.62% + 29.41% + 27.78% = 104.81% (exactly: 104.8086%). That’s this market’s overround. Since the total should be exactly 100% on a “fair” market, the surplus of 4.81 percentage points is the bookmaker’s margin, spread across all three possible outcomes.
Where exactly that surplus disappears to — proof in zloty
To show that those 4.81 percentage points are a real, measurable profit for the bookmaker, not just an abstract number, let’s calculate a balanced “book”: stakes spread exactly in proportion to each outcome’s implied probability, so the payout on any winning outcome comes to roughly 1,000 zl.
| Outcome | Odds | Stake (zl) | Payout if it wins (zl) |
|---|---|---|---|
| Home | 2.10 | 476.19 | 476.19 x 2.10 = 1000.00 |
| Draw | 3.40 | 294.12 | 294.12 x 3.40 = 1000.01 |
| Away | 3.60 | 277.78 | 277.78 x 3.60 = 1000.01 |
Total stakes: 476.19 + 294.12 + 277.78 = 1,048.09 zl. Regardless of which of the three outcomes actually happens, the bookmaker always pays out about 1,000.00 zl to the winning side (the odd penny differences in the table are rounding on the individual stakes). Bookmaker’s profit: 1,048.09 − 1,000.00 = 48.09 zl — and that’s true regardless of the match result. This is exactly the mechanism that makes margin a structural, guaranteed edge for the bookmaker, not a bet on any particular outcome.
Two different numbers: margin on “fair” value vs margin on turnover
Here’s a subtlety many articles about margin skip. That 48.09 zl of profit can correctly be expressed two different ways:
- As a % of the “fair” payout (1,000 zl): 48.09 ÷ 1,000 = 4.809% ≈ 4.81% — exactly the same number as the overround minus 100%. This confirms the two calculations agree.
- As a % of total turnover, i.e. all stakes combined (1,048.09 zl): 48.09 ÷ 1,048.09 = 0.045875 ≈ 4.59%. That’s the bookmaker’s real share of every zloty players actually stake on that market — slightly lower than 4.81%, because it’s measured against a bigger base.
Both numbers are correct — they simply differ in what base you’re calculating the percentage from. In practice, when someone says “4.81% margin”, they usually mean the first definition (overround minus 100%) — it’s the more convenient one for quickly comparing prices between bookmakers, since it’s calculated directly from the odds themselves, without needing to simulate a balanced book.
Why a lower overround is better for the bettor
The lower the sum of implied probabilities above 100%, the lower the structural cost of playing that market. A market with 102% overround is statistically better value than one with 108% — with the same hit rate, a bettor who consistently plays lower-margin markets loses less over the long run. That’s why comparing odds between licensed bookmakers on the same market makes sense even when the difference looks like a few groszy on a single bet — across hundreds of slips over a season, those differences add up. The full, verified line-up of all licensed operators to start that comparison with is in our ranking of licensed bookmakers.
Why overround differs between markets
Overround isn’t one fixed number for a given bookmaker — it varies by market. A simple, liquid, high-volume market (main 1X2 on a popular league) usually has a lower overround than a niche market (e.g. exact score or corner count) — lower volume means more risk on a single event for the operator, which it compensates for with a higher margin. For the same reason, markets on less popular competitions (lower leagues, less-followed sports) usually carry higher overround than markets on the most popular matches and leagues.
Overround on a two-outcome market — the simpler case
The mechanism works identically on markets with two possible outcomes (e.g. picking the winner of a tennis match). At odds of 1.80 and 2.00: implied probabilities are 55.56% and 50.00% respectively, summing to 105.56%, an overround of 5.56%. The fewer outcomes a market has, the easier it is to spot the margin “with the naked eye” — on a three-outcome 1X2 market it’s spread across three prices at once, so it can be less obvious than on a simple two-outcome contest.
Overround and accumulators — why the margin multiplies
Everything above concerns a single event. When you combine several events into one accumulator slip, each leg’s overround multiplies with the others’ — it doesn’t add. The effect is much stronger than most bettors expect: a margin of roughly 5% on a single event translates, in a four-leg accumulator, to a combined margin above 20% — not 5%, and not a simple sum of 4×5%. We show the full, step-by-step calculation in accumulator margin.
Overround and tax — two separate layers of cost
Overround is a cost the bookmaker builds into the price itself. In Poland there’s a second, completely separate layer on top: a 12% betting tax, charged on the stake and remitted by the operator. Both layers work independently, and both reduce the real payout relative to a “fair”, margin-free market — we calculate the combined effect step by step in Poland’s 12% betting tax and your real payout.
A second fully worked example — a two-outcome over/under market
The same mechanism on another, equally illustrative market: “over 2.5 goals” at odds of 1.95, “under 2.5 goals” at odds of 1.85. Implied probabilities (full precision): 1 ÷ 1.95 = 0.512821, 1 ÷ 1.85 = 0.540541. Total: 0.512821 + 0.540541 = 1.053361, i.e. 105.34%, so an overround of 5.34%. Balanced book (stakes proportional to implied probability, scaled to a payout of about 1,000 zl): Over 1,000 ÷ 1.95 = 512.82 zl, Under 1,000 ÷ 1.85 = 540.54 zl, together 1,053.36 zl of turnover. Bookmaker’s profit regardless of the result: 1,053.36 − 1,000.00 = 53.36 zl — as a % of fair value (1,000 zl): 53.36 ÷ 1,000 = 5.336% ≈ 5.34% (matches overround minus 100%); as a % of total turnover (1,053.36 zl): 53.36 ÷ 1,053.36 ≈ 5.07%. Exactly the same pattern as the 1X2 example above — both numbers correct, different base.
The same match, two bookmakers — low margin vs high margin
To show that a difference in overround isn’t an abstraction but real zloty in your account, let’s take one shared starting point: a purely illustrative match where the “fair” (margin-free) probabilities are Home 45%, Draw 27%, Away 28% (summing to exactly 100%). Fair odds, with no margin at all: Home 1 ÷ 0.45 = 2.2222, Draw 1 ÷ 0.27 = 3.7037, Away 1 ÷ 0.28 = 3.5714.
Now the same match at two different licensed bookmakers, who add different amounts of margin to those same fair probabilities:
| Outcome | Fair odds (0% margin) | Bookmaker A — low margin (~2.9%) | Bookmaker B — high margin (~10%) |
|---|---|---|---|
| Home | 2.2222 | 2.16 | 2.02 |
| Draw | 3.7037 | 3.60 | 3.37 |
| Away | 3.5714 | 3.47 | 3.25 |
Checking the overround: at Bookmaker A, the sum of implied probabilities (1÷2.16 + 1÷3.60 + 1÷3.47) = 46.30% + 27.78% + 28.82% = 102.89%, a margin of 2.89%. At Bookmaker B, the sum (1÷2.02 + 1÷3.37 + 1÷3.25) = 49.50% + 29.67% + 30.77% = 109.95%, a margin of 9.95%.
The effect in zloty — stake 100 zl on Home at both operators: at Bookmaker A the winning payout = 100 x 2.16 = 216.00 zl; at Bookmaker B = 100 x 2.02 = 202.00 zl. Difference: 216.00 − 202.00 = 14.00 zl on exactly the same bet, at exactly the same risk. Expressed as a percentage of the fair-odds value (222.22 zl): Bookmaker A gives you 216.00 ÷ 222.22 ≈ 97.20% of fair value, Bookmaker B only 202.00 ÷ 222.22 ≈ 90.90% — a 6.3-percentage-point gap in value on a single bet. On the Draw the gap in zloty is even bigger: 360.00 zl vs 337.00 zl, i.e. 23.00 zl on a 100 zl stake; on Away, 347.00 zl vs 325.00 zl, i.e. 22.00 zl.
This is exactly why comparing odds between licensed bookmakers on the same market, before any sizeable bet, has real, measurable value — it isn’t a cosmetic difference of a few groszy, it’s tens of zloty on every 100 zl staked, repeatedly, on every slip.
What a “no-vig” market means — and why it barely exists in practice
A theoretical “no-vig” market is one where the sum of implied probabilities of all outcomes comes to exactly 100% — the bookmaker would have no structural edge at all, and would only profit by chance from beating the market. In practice, such a market doesn’t exist at a licensed, commercial bookmaker — even operators marketing themselves as “low-margin” carry a small but non-zero overround, because that margin covers operating costs, model risk and company profit. The “no-vig” number is still analytically useful, though — it’s exactly what normalised implied probability approximates, described in implied probability.
Overround through the season and between leagues
Overround also isn’t constant over time for the same bookmaker on the same league. Early in a season, when an operator’s model has less current data on team form, the margin on less obvious matches tends to be higher than mid-season, once form and head-to-head history are richer. Similarly, lower leagues and less-followed competitions usually carry a higher overround than the top European leagues — lower betting volume means more risk on a single market for the bookmaker, which it compensates for with a higher margin.
How to check the overround of any market yourself
- Write down the odds for every possible outcome on a given market at one bookmaker, at the same moment.
- For each price, calculate 1 ÷ odds — that’s the implied probability of that outcome.
- Sum all the values you get.
- Subtract 100% from the total — the result is the overround, i.e. that operator’s margin on that specific market.
- Repeat for the same market at another licensed bookmaker and compare — a lower value means statistically better prices.
For faster calculations on your own odds (not just the examples in this article), you can also use our betting calculators.
Common mistakes interpreting overround
- Confusing overround with the bookmaker’s profit on one specific bet — overround is a structural, statistical edge over the long run, not a guarantee of profit on a single slip.
- Assuming a fixed overround for a given operator — it varies between markets and matches, it isn’t one universal number.
- Confusing overround (% of fair value) with share of turnover — both numbers are correct, but measured against a different base, as shown above with 4.81% vs 4.59%.
- Ignoring overround on accumulators — the compounding effect on margin is often much bigger than the single-leg margin would suggest.
What this means for you in practice
You don’t need to calculate overround on every slip you bet — but it’s worth doing at least once, on your usual market, to have a reference point. A concrete plan: (1) pick the market you bet on most often (e.g. 1X2 on your favourite league); (2) write down the odds for every outcome at your usual bookmaker; (3) calculate the overround with the same formula as above; (4) repeat the same thing at one or two other licensed operators, on the same match, at the same time; (5) if the difference exceeds 2–3 percentage points, as in the Bookmaker A vs B example above, it’s worth considering moving your regular betting to the operator with the lower overround — that directly affects how much stays in your pocket over the long run, regardless of how accurate your picks are. The line-up of all licensed operators worth starting that comparison with is in our ranking of licensed bookmakers.
Frequently Asked Questions
What exactly is overround?
It’s the sum of implied probabilities (1 ÷ odds) of every possible outcome on a market, expressed as a percentage. On a fair, “zero-margin” market it would be exactly 100%. With every bookmaker it exceeds 100% — the surplus above 100% is the overround, i.e. the built-in margin.
How much overround do licensed bookmakers in Poland actually carry?
It varies between operators and between markets — we don’t publish one universal figure here, because it changes match to match and market to market. The rule, though, is constant: the simpler and more liquid a market (main 1X2, a popular league), the lower the overround tends to be. Operators known for a lower margin, i.e. statistically better prices, include AdmiralBet, Betclic and Superbet.
Does a lower overround always mean a better bookmaker?
For long-term value — yes, a lower margin statistically means a lower cost of play over time. It isn’t the only selection criterion, though — market breadth, app quality and payout speed matter too. See our full methodology for rating bookmakers.
Is overround the same as the bookmaker’s share of total turnover?
Not quite — they’re two different, related numbers. Overround is measured against the “fair” 100% baseline (e.g. 104.81% = 4.81% margin above fair value). The bookmaker’s real share of profit in the ENTIRE amount staked (on a balanced book) is somewhat lower, because it’s measured against a bigger base — in the worked example below that’s 4.59%, not 4.81%. We calculate both figures step by step in this article.
Does overround apply to accumulators too?
Yes, and there the effect is stronger — the overround of individual legs multiplies (not adds) when combined into one slip. We show the full calculation of how much that really adds to a multi-leg slip’s margin in accumulator margin.