How Betting Odds Work
Decimal, fractional and American odds — what they mean, how to convert them to probability, and how to spot the operator margin.
Decimal odds (Cyprus/European standard)
Example: 2.50. Multiply stake by odds for total payout. €10 × 2.50 = €25 (€15 profit).
Probability
Implied probability = 1 / odds. 1 / 2.50 = 40%.
Fractional odds (UK)
5/2 means €5 profit for every €2 staked. Equals decimal 3.50.
American odds (US)
+150 means €150 profit on a €100 stake. -200 means €100 profit on a €200 stake.
The operator margin (overround)
Add implied probabilities of every outcome. If it exceeds 100%, the excess is the margin. Example: 1X2 at 2.00 / 3.50 / 4.00 → 50% + 28.6% + 25% = 103.6% → 3.6% margin.
The lower the margin, the better value for the bettor.