Odds, probability and the bookmaker's margin
Decimal, fractional and American odds, the probability inside a price, and how a book builds in its margin.
Every price at a sportsbook is a probability in disguise, plus the book's cut. Read that and the rest of this guide follows.
Three ways to write the same odds
| Format | Example | What it means for a 10 stake |
|---|---|---|
| Decimal | 2.50 | Returns 25.00 in total if it wins (10 back + 15) |
| Fractional | 3/2 | Wins 15 on top of the 10 staked |
| American | +150 | Wins 150 per 100 staked |
We use decimal odds everywhere: one number, and the return is just stake × odds. To convert, fractional a/b is 1 + a/b in decimal, and American +X is 1 + X/100 (for -X, it's 1 + 100/X).
The probability inside a price
The implied probability of a decimal price is 1 / odds.
2.00→ 1 / 2.00 = 50%1.25→ 1 / 1.25 = 80%4.00→ 1 / 4.00 = 25%
It is the chance at which the bet would break even. If you think the real chance is higher than the implied one, the price is generous; lower, and it's mean.
The bookmaker's margin
Add up the implied probabilities of every outcome of one market at one book and you get more than 1. The excess is the book's margin (also called the overround or vig).
That margin is why betting one book on every outcome always loses a little. A surebet turns it around: you take each outcome at the book that prices it best, and if the best prices are generous enough, the sum drops below 1. That's the next section: What is a surebet?
Quick reference
- Return = stake × decimal odds
- Implied probability = 1 / decimal odds
- One book's market: sum of 1/odds above 1 = the book's margin
- Best prices across books: sum below 1 = a surebet
See it on real data
A free account shows up to 3 live surebets, each 0.99% or less, every leg re-checked at its book. No card needed.