4. Margin and overround

How does the bookmaker make money?

In chapter 1 you saw the principle; now comes the machinery. In the bookmaker’s ideal world its book is balanced: enough money arrives on every outcome that whichever comes in, the payout is roughly the same, and the margin stays in its pocket as a sure profit. In that case the bookmaker really does not play against you: the losers pay the winners, and it pockets the difference.

In reality the book is rarely perfect, so the bookmaker steers the money by moving the odds: whichever side too much money flows to, it makes cheaper (lowers the odds), and makes the other more attractive. And where it is more uncertain in its pricing, it uses a larger margin from the outset. That is why the margin depends on the market: small on the main markets of big, liquid leagues, large on side markets and in small leagues (chapter 7).

The margin is not a fee that is invoiced but a cost hidden in the price. That is exactly why you need to be able to calculate it: otherwise you don’t even know how much you are paying.

Overround calculation on 2- and 3-outcome markets

The overround is the sum of the implied probabilities of all outcomes of a market, and the margin is its part above 100%. The calculation: add up the 1/odds values. On a two-outcome market where both sides are 1.90: 52.6% + 52.6% = 105.3%, so the margin is 5.3%.

On a three-outcome market the same way. In the 1X2 example of chapter 3 (2.10 / 3.40 / 3.60) the sum is 104.8%, the margin 4.8%. At a typical soft-bookmaker price (2.00 / 3.30 / 3.50) the sum is already 108.9%, at a sharp bookmaker (2.10 / 3.45 / 3.70) only 103.6%. On the same match there is more than 5 percentage points of cost difference between the two prices.

The overround also gives the market’s long-run payback: the ratio is 100 / overround. The 105.3% market pays back 95.0% of the money staked in the long run, the 108.9% one only 91.8%.

Your expected loss without an edge: 100% minus the payback. On the 5.3%-margin market it is 5.0%, on the 8.9% one 8.2%, per single bet.

Margin removal: even vs odds-proportional method

To get the fair price the margin must be removed, and here is the twist: there are several methods, and it matters which one you use. You already know the proportional (multiplicative) method from chapter 3: divide each implied probability by the sum. The even (additive) method instead subtracts the same number of percentage points from every outcome, spreading the margin equally.

The two really diverge at high odds. The same market (1.30 / 5.50 / 9.00, margin 6.2%) with the two methods:

The truth is typically between the two: bookmakers load proportionally more margin onto low-chance outcomes (favourite-longshot bias, chapter 35), so the proportional method overestimates the longshot’s fair chance and the even one punishes it too harshly. The common middle solution is the power method, which raises the probabilities to a common exponent until the sum is exactly 100%. For a quick calculation the proportional one is plenty good (the platform’s calculator gives this too, chapter 43), but remember: the fair price is an estimate, and method-dependent.

Comparing bookmakers’ margins

How much margin counts as a lot? Sharp bookmakers work with 2–3% on the main markets, soft bookmakers typically with 5–8%, and on side markets and in small leagues even 8–12% is not rare. This is not theory: the platform’s Market efficiency page shows the real, measured average margin by league (chapter 44).

Calculate what this means on an annual scale. With 10 bets of €100 a week, at a 5% average margin, and an annual turnover of €52,000, your built-in cost is roughly €2,600. The same at a 2.5% margin is €1,300. A difference of €1,300 a year purely from where and on what market you bet. That is why the first defence is seeking the better price (line shopping, chapter 13), long before you think about any winning tip.

The margin is the one cost you are guaranteed to pay, after every bet, whether you won or lost. Reducing it is therefore the most reliable return you can achieve as a bettor.