Take three prices for a match: 1 (home) 2.20, X (draw) 3.40, 2 (away) 3.30. Convert each into a “probability” as 1 divided by the price, add them up — and you get more than 100%. That is not a mistake. That is the bookmaker’s margin, known as the overround.
Where the surplus comes from
A price is a probability in disguise. Odds of 2.00 correspond to 50% (1 / 2.00 = 0.50), odds of 4.00 to 25%, and so on. If a bookmaker priced events perfectly fairly, the probabilities of all possible outcomes would add up to exactly 100%.
But then the operator would earn nothing. So every price is shaded slightly down — a marginally worse price than a fair valuation would give. Add those shadings together and you land above 100%. The surplus over the hundred is the overround (also called the margin, or the bookmaker’s edge).
Let us calculate it on our example
1 / 2.20 = 0.4545 1 / 3.40 = 0.2941 1 / 3.30 = 0.3030 total = 1.0516 → 105.16%
The overround is those 5.16 percentage points above the hundred. Put plainly: the market pretends to cover 105% of the event while it really covers 100%. That difference stays in the operator’s pocket regardless of how the match ends — because it is spread across all outcomes at once.
How large is it in practice
The size of the overround depends on the operator, the market and the profile of the event. On the most liquid markets of the biggest leagues margins can be thin; on niche ones they are far fatter, because less competition and more uncertainty let the operator add to the price. Side markets (total goals, corners, cards) usually carry a thicker margin than the basic 1X2. Rule of thumb: the less liquid and more exotic the market, the bigger the overround — and the Ekstraklasa and lower divisions are exactly those less-bet markets.
Overround versus tax — two different costs
They are easy to confuse, so let us separate them: the overround sits inside the price itself (the operator’s margin), while the 12% tax comes off your stake (a levy for the state). They are independent and they stack. First the price already comes seasoned with margin, and then the state takes its 12% of what you deposit. Two layers of cost, one on top of the other — which is why the real distance to break-even is longer than either of them suggests alone.
Why we need this in analysis
The overround is an obstacle for the bettor, but for an analyst it is also information to be stripped out. To compare a probability from the model with what the market “really” thinks, you first have to clean the margin out of the price — that is, redistribute those surplus 5% back across the outcomes so the total returns to 100%. That operation is called devigging; we devote a separate piece to it.
Until you do that, you are comparing your assessment with a price that has been seasoned with margin, and it is easy to imagine an edge that is not there. That is why the overround is not a footnote for us — it is the first step of every honest model-versus-market comparison.
Educational material. Not advice on taking part in betting. 18+.