The three odds formats
Decimal odds (2.50) are the most common in Europe: they show the total return per €1 bet, stake included. Fractional odds (3/2), common in the UK, show the profit relative to the stake. American odds (+150 or −200) show what a $100 bet wins, or how much you must bet to win $100.
The implied probability of odds
To see what chance the bookmaker gives an outcome, divide 1 by the decimal odds: odds of 2.50 mean 40%. If you think the real probability is higher, the odds are good value for you.
The bookmaker's margin
Add up the implied probabilities of every outcome in a match: the total is always above 100%. The excess is the bookmaker's margin. The lower it is, the better the odds for the bettor.
Frequently asked questions
How do I convert 1.80 to American odds?
Decimal odds below 2 give negative American odds: −100 ÷ (1.80 − 1) = −125.
Do big odds mean a big win?
Yes, but with a low probability: odds of 10 mean about a 10% chance according to the bookmaker.
Why do odds change?
They move with bettors' stakes and new information: injuries, line-ups, how a live match unfolds.