How betting odds work: decimal, American and implied probability
Every odds format is the same thing wearing different clothes: a price that implies a probability. If you can convert odds to probability in your head, you can instantly tell whether a line is generous or greedy.
Decimal odds
The world standard outside the US. Decimal odds of 2.50 mean a winning 10-unit bet returns 25 (stake included). Implied probability = 1 ÷ 2.50 = 40%.
American odds
Positive numbers (+150) show profit on a 100 stake: bet 100, win 150. Implied probability = 100 ÷ (150 + 100) = 40%. Negative numbers (−200) show the stake needed to win 100: implied probability = 200 ÷ (200 + 100) = 66.7%.
The vig: why the book starts ahead
Add up the implied probabilities of every outcome in a market and you'll get more than 100% — usually 104–108% in a two-way market. That excess is the bookmaker's margin (the "vig"). A coin flip priced fairly would be 2.00 / 2.00; a real book offers 1.91 / 1.91. Both sides are slightly overpriced, and the gap is the operator's edge.
The only habit that matters
- Convert the odds to implied probability.
- Ask: do I honestly think the true chance is higher than that?
- If not, there is no bet — no matter how much you like the team.
That discipline — betting a number, not a feeling — is the difference between punting and edge-seeking. Our daily brief flags lines where the implied probability looks out of step with the evidence.