What the margin is
Odds are a promise of a payout: 2.00 means that every unit staked comes back as two. For an outcome with a 50% chance, the fair odds are exactly 2.00. A bookmaker offers a little less, say 1.90 on both outcomes, and keeps the difference.
Add up the probabilities built into the odds of all the outcomes (1 / odds): a fair line comes to exactly 100%, a bookmaker's line to more. That excess is the margin, also called the overround. In our data — 43,352 matches in 27 football leagues since 2017 — an average bookmaker's margin is 5.8%. The sharp line (bookmakers who take big bets from professionals and keep a low margin) has 3.2%. And if you take the best price on the market for every outcome, the margin almost disappears: 0.3%.
How much betting on everything loses
Bet on every outcome and you lose almost exactly the margin: −7.1% with an average bookmaker. This is the baseline for any strategy: if it does not win back that 7%, it loses just like betting at random.
Where the margin is bigger
The margin is not spread evenly. Bets on a home win end at −6.3%, on a draw at −5.6%, on an away win at −9.3%. The odds on unlikely outcomes are trimmed the most — more on that in the article on favourites and underdogs.
In hockey and basketball
In hockey and basketball people usually bet on the winner including overtime (and, in hockey, the shoot-out): two outcomes instead of three. The margin is there all the same:
| Sport | Matches | Margin | Betting on everything | At the best price |
|---|---|---|---|---|
| Football | 43,352 | 5.8% | −7.1% | −1.1% |
| Hockey | 3,534 | 6.3% | −7.3% | −5.4% |
| Basketball | 2,721 | 5.0% | −6.5% | −2.9% |
What follows from this
- Any strategy first has to win back the margin, and only then can it make a profit.
- How you spread your stakes does not remove the margin: chasing losses, the ladder and progressions do not change it.
- It can be reduced: take the best price (−1.1% instead of −7.1%) and stay away from bets where the margin is highest.