How betting odds work: fractions, decimals and the bookmaker’s margin
Read any odds format, turn odds into a probability, and see the built-in margin that means the bookmaker wins in the long run.
Odds tell you two things: how much you’ll win, and how likely the bookmaker thinks an outcome is. Once you can read them, you can see exactly how the bookmaker makes money.
Three ways to show the same price
- Fractional (5/2): win €5 for every €2 staked, plus your stake back.
- Decimal (3.50): your total return for each €1, including the stake. €10 at 3.50 returns €35.
- American (+250): win $250 on a $100 stake. A minus sign (−150) shows how much you must stake to win $100.
Odds as a probability
For decimal odds, divide 1 by the odds. Odds of 2.00 imply a 50% chance; 4.00 implies 25%. This “implied probability” is the bookmaker’s view, plus its margin.
The bookmaker’s margin
Add up the implied probabilities for every outcome in a market. In a fair market they’d add up to 100%. In practice they come to more — say 105%. That extra 5% is the margin, and it’s why the average bettor loses over time. It works much like the house edge in a casino.
Why accumulators are expensive
Each selection in an accumulator carries its own margin, and they multiply. A four-leg acca can easily carry a margin of 15–20% — great headlines, poor value.
What it means for you
Betting can be fun, but it isn’t a way to make money. Keep stakes small, set a deposit limit, and treat each bet as the price of an evening’s entertainment.