Betting Odds Explained: Decimal, Fractional and American
2.50, 3/2 and +150 are the same price written three ways. How to read each format, turn odds into a probability, and spot the bookmaker's margin.
Three ways to write one price
| Decimal | Fractional | American | €10 stake returns |
|---|---|---|---|
| 1.50 | 1/2 | −200 | €15 |
| 2.00 | 1/1 (evens) | +100 | €20 |
| 2.50 | 3/2 | +150 | €25 |
| 4.00 | 3/1 | +300 | €40 |
- Decimal (Europe, Canada, Australia): the total return per 1 staked, including the stake.
- Fractional (UK): profit relative to stake — 3/2 means 3 profit for every 2 staked.
- American: +150 is the profit on a 100 stake; −200 is how much you stake to win 100.
Odds as a probability
Decimal odds convert to an implied probability with one division: 1 ÷ odds. Odds of 2.50 imply 1 ÷ 2.50 = 40%. Odds of 1.50 imply about 67%.
The bookmaker's margin
Add up the implied probabilities of every outcome in a market and you will get more than 100%. For a football match priced 2.10 / 3.40 / 3.60, that is 47.6% + 29.4% + 27.8% = 104.8%. The extra 4.8% is the overround — the bookmaker's built-in margin. The lower it is, the better the prices.
Odds are an opinion about probability — plus a margin. They are not a prediction of what will happen.
