The three odds formats
- Fractional odds (5/2) are traditional in the UK and Ireland. They show profit relative to stake: 5/2 pays £5 profit for every £2 staked.
- Decimal odds (3.50) show the total return per £1, including your stake. They're common across Europe and on betting exchanges.
- American odds (+250 / -200) are used in the US. A plus number is the profit on a £100 stake; a minus number is the stake needed to win £100.
All three can describe the same price: 5/2, 3.50 and +250 are identical odds.
How to convert odds yourself
- Fractional to decimal: divide the top number by the bottom and add 1. 5/2 = 2.5 + 1 = 3.50.
- Decimal to fractional: subtract 1 and turn it into a fraction. 3.50 − 1 = 2.5 = 5/2.
- Decimal to American: at 2.0 or above, (decimal − 1) × 100 with a plus sign. Below 2.0, −100 ÷ (decimal − 1).
What implied probability tells you
Implied probability is 1 ÷ decimal odds. Odds of 3.50 imply a 28.57% chance. Add up the implied probabilities for every outcome in a market and you'll usually get more than 100%. The extra is the bookmaker's margin, so odds always pay slightly less than the true chance would suggest.
